add/remove context menu (requires tabs permission)

background.js

chrome.tabs.onUpdated.addListener(function(tabId,changeInfo,tab){ //onUpdated should fire when the selected tab is changed or a link is clicked.
  chrome.tabs.getSelected(null,function(tab){
    if (tab.url=='http://techmeme.com/')
      chrome.contextMenus.create(contextMenuSpec);
    else
      chrome.contextMenus.removeAll();
  }); 
});

var contextMenuSpec = { 
  title: 'Add to favorites',
  contexts: ['all'],
  onclick: function(info, tab) {
    shortenLink(info, tab, {method:'bitly'});
  }
};