I would suggest to go with post (tweet, rss) persistence as soon as possible, because of several reasons:
a) it would allow us to use more complex processing before posting them.
Let's say if we would like to change shortened urls in tweeter posts to real ones, we would need to make request to every single url in tweet post, to check if it does redirection. While it is possible to make that at run time, doing some fancy lazy replacement, it would be easier to do that before persisting and then just select posts needed to display from persistence.
May be use some mapping from original categories to dotnetgroup categories, like ITishnikai => Podcasts.
May be for some spam filters.
b) usually rss feeds contains only 10-20 latest posts, so we could keep them all.
c) it would be possible to attach comments/votes/etc. to posts, I mean content, that is generated on dotnetgroup site, also it would be used to bind several sources to single member (idea would be - my footprint on dotnetgroup site = my cv :).
I would move Facebook authentication/comments to second release, and move stackoverflow to third.
Also, I'm missing UI sketches (http://www.balsamiq.com/), that would be fastest way to communicate, where are we going.