Somewhat improved version of Fake News Alert extension 0.1.2 from https://chrome.google.com/webstore/detail/fake-news-alert/aickfmgnhocegpdbfnpfnedpeionfkbh/related – to mark links directly at Facebook before you even click through them.

alert.js

// alert.js

const stripes = document.createElement('div');
stripes.style.cssText = `
  height: 20px;
  width: 100%;
  position: fixed;
  z-index: 99999;
  opacity: 0.7;
  pointer-events: none;
  background-image: repeating-linear-gradient(
    -45deg, #000, #000 20px, #df0 20px, #df0 40px
  );
`;

const banner = document.createElement('span');
banner.textContent = `Claims on this site are often false or misleading!`;
banner.style.cssText = `
  font-family: 'Lucida Console', Monaco, monospace;
  color: white;
  margin: auto;
  background: black;
  display: block;
  width: 500px;
  height: 20px;
  line-height: 20px;
  text-align: center;
  font-size: 14px;
`;

stripes.appendChild(banner);

document.body.insertBefore(stripes, document.body.firstChild);

facebook.js

// facebook.js

const normalize = (domain) => domain.toLowerCase().replace(/^www\./, '');

const isMatch = (domain) => {
  if (!isMatch.domains) {
    const manifest = chrome.runtime.getManifest();
    isMatch.domains = manifest.content_scripts[0].matches.map(s =>
      normalize(s.replace(/^\*:\/\/(?:\*\.)?([^/]*).*/, '$1'))
    );
  }
  return isMatch.domains.indexOf(domain) !== -1;
};

const flag = (div) => {
  div.classList.add('fake-news-checked'); // only check each link once

  let domain = div.firstChild.nodeValue;
  if (!domain) { // also handle <span class="highlightNode">domain</span>:
    domain = div.firstElementChild.firstChild.nodeValue;
  }
  if (!isMatch(normalize(domain))) {
    // console.info(`skipped ${domain}`);
    return;
  }
  // console.info(`flagged ${domain}`);

  const root = div.parentNode.parentNode.parentNode.parentNode;

  const stripes = document.createElement('div');
  stripes.style.cssText = `
    position: absolute;
    height: 28px;
    bottom: 0px;
    right: 0;
    left: 0;
    pointer-events: none;
    background: repeating-linear-gradient(
      -45deg, #000, #000 20px, #df0 20px, #df0 40px
    );
  `;

  const warn = document.createElement('div');
  warn.textContent = 'SITE OFTEN FALSE OR MISLEADING';
  warn.style.cssText = `
    right: 8px;
    bottom: 8px;
    text-align: right;
    position: absolute;
    font-size: 11px;
    color: red;
    text-shadow:
      -1px -1px 0 #000,
      -1px  0   0 #000,
      -1px  1px 0 #000,
       0   -1px 0 #000,
       0    1px 0 #000,
       1px -1px 0 #000,
       1px  0   0 #000,
       1px  1px 0 #000;
  `;
  stripes.appendChild(warn);

  root.insertBefore(stripes, root.firstChild);
};

flagNew = () => {
  document.querySelectorAll('._6lz._6mb:not(.fake-news-checked)').forEach(flag);
};

// initial page sweep
flagNew();

// capture infinite-scroll-added content, too
setInterval(flagNew, 1e3);

manifest.json

{ "browser_action":
  { "default_icon": "icons/icon48.png"
  }
, "content_scripts":
  [ { "js":
      [ "alert.js"
      ]
    , "matches":
      [ "*://*.100percentfedup.com/*"
      , "*://*.21stcenturywire.com/*"
      , "*://*.70news.wordpress.com/*"
      , "*://*.abcnews.com.co/*"
      , "*://*.activistpost.com/*"
      , "*://*.addictinginfo.org/*"
      , "*://*.americannewsx.com/*"
      , "*://*.samericannews.com/*"
      , "*://*.anonnews.co/*"
      , "*://*.associatedmediacoverage.com/*"
      , "*://*.beforeitsnews.com/*"
      , "*://*.beingliberal.org/*"
      , "*://*.bigamericannews.com/*"
      , "*://*.bigpzone.com/*"
      , "*://*.bipartisanreport.com/*"
      , "*://*.www.bizpacreview.com/*"
      , "*://*.bluenationreview.com/*"
      , "*://*.www.breitbart.com/*"
      , "*://*.thebostontribune.com/*"
      , "*://*.cap-news.com/*"
      , "*://*.christwire.org/*"
      , "*://*.chronicle.su/*"
      , "*://*.civictribune.com/*"
      , "*://*.coasttocoastam.com/*"
      , "*://*.collectiveevolution/*"
      , "*://*.consciouslifenews.com/*"
      , "*://*.conservativeoutfitters.com/*"
      , "*://*.wideawakeamerica.com/*"
      , "*://*.countdowntozerotime.com/*"
      , "*://*.counterpsyops.com/*"
      , "*://*.creambmp.com/*"
      , "*://*.dailybuzzlive.com/*"
      , "*://*.dailycurrant.com/*"
      , "*://*.dailywire.com/*"
      , "*://*.dcclothesline.com/*"
      , "*://*.dcgazette.com/*"
      , "*://*.denverguardian.com/*"
      , "*://*.derfmagazine.com/*"
      , "*://*.disclose.tv/*"
      , "*://*.drudgereport.com.co/*"
      , "*://*.duffleblog.com/*"
      , "*://*.duhprogressive.com/*"
      , "*://*.embols.com/*"
      , "*://*.empireherald.com/*"
      , "*://*.empirenews.com/*"
      , "*://*.endingthefed.com/*"
      , "*://*.enduringvision.com/*"
      , "*://*.fprnradio.com/*"
      , "*://*.thefreethoughtproject.com/*"
      , "*://*.geoengineeringwatch.org/*"
      , "*://*.govtslaves.info/"
      , "*://*.gulagbound.com/"
      , "*://*.hangthebankers.com/*"
      , "*://*.humansarefree.com/*"
      , "*://*.huzlers.com/*"
      , "*://*.ifyouonlynews.com/*"
      , "*://*.infowars.com/*"
      , "*://*.intellihub.com/*"
      , "*://*.inquisitor.com/*"
      , "*://*.jonesreport.com/*"
      , "*://*.lewrockwell.com/*"
      , "*://*.liberalamerica.org/*"
      , "*://*.libertytalk.fm/*"
      , "*://*.libertyunyielding.com/*"
      , "*://*.libertyvideos.org/*"
      , "*://*.mediamass.net/*"
      , "*://*.megynkelly.us/*"
      , "*://*.msnbc.com.co/*"
      , "*://*.msnbc.website/*"
      , "*://*.nahadaily.com/*"
      , "*://*.nationalreport.net/*"
      , "*://*.naturalnews.com/*"
      , "*://*.nbc-news.net/*"
      , "*://*.ncscooper.com/*"
      , "*://*.newcenturytimes.com/*"
      , "*://*.newsexaminer.net/*"
      , "*://*.news-hound.com/*"
      , "*://*.newsbiscuit.com/*"
      , "*://*.newsbuzzdaily.com/*"
      , "*://*.newslo.com/*"
      , "*://*.newsmutiny.com/*"
      , "*://*.newswatch28.com/*"
      , "*://*.opposingviews.com/*"
      , "*://*.newswire-24.com/*"
      , "*://*.now8news.com/*"
      , "*://*.nowtheendbegins.com/*"
      , "*://*.occupydemocrats.com/*"
      , "*://*.www.pakalertpress.com/*"
      , "*://*.politicalblindspot.com/*"
      , "*://*.politicalears.com/*"
      , "*://*.politicalo.com/*"
      , "*://*.politicususa.com/*"
      , "*://*.prisonplanet.com/*"
      , "*://*.private-eye.co.uk/*"
      , "*://*.projectveritas.com/*"
      , "*://*.react365.com/*"
      , "*://*.realfarmacy.com/*"
      , "*://*.realnewsrightnow.com/*"
      , "*://*.redflagnews.com/*"
      , "*://*.redstate.com/*"
      , "*://*.rilenews.com/*"
      , "*://*.satiratribune.com/*"
      , "*://*.theblaze.com/*"
      , "*://*.thefreethoughtproject.com/*"
      , "*://*.newyorker.com/humor/borowitz-report/*"
      , "*://*.other98.com/*"
      , "*://*.www.reporter.bz/*"
      , "*://*.www.thestatelyharold.com/*"
      , "*://*.www.thedailysheeple.com/*"
      , "*://*.www.thenewsnerd.com/*"
      , "*://*.therundownlive.com/*"
      , "*://*.theuspatriot.com/*"
      , "*://*.truthfrequencyradio.com/*"
      , "*://*.twitchy.com/*"
      , "*://*.unconfirmedsources.com/*"
      , "*://*.usasupreme.com/*"
      , "*://*.blastingnews.com/*"
      , "*://*.usuncut.com/*"
      , "*://*.www.veteranstoday.com/*"
      , "*://*.wakingupwisconsin.com/*"
      , "*://*.winningdemocrats.com/*"
      , "*://*.witscience.org/*"
      , "*://*.www.wnd.com/*"
      , "*://*.worldnewsdailyreport.com/*"
      , "*://*.worldtruth.tv/*"
      , "*://*.zerohedge.com/*"
      ]
    }
  , { "js":
      [ "facebook.js"
      ]
    , "matches":
      [ "*://*.facebook.com/*"
      ]
    }
  ]
, "default_locale": "en"
, "description": "Alerts you if you are viewing a fake news site"
, "homepage_url": "http://nymag.com/"
, "icons":
  { "128": "icons/icon128.png"
  }
, "manifest_version": 2
, "minimum_chrome_version": "22.0.0.0"
, "name": "Fake News Alert"
, "version": "0.2.0"
}

Subsequent better update in https://github.com/bfeldman/fake-site-alert/pull/19 – now that I found the extension's origin source code.