This is very well made, great job. A few things:

- Why do I have to choose the best square thumbnail picture? Facebook already provides you with pic_square through their API.

- Profile thumbnails are not loading unless I view the user's profile page first. Here's an example, where I have clicked through to Adam's profile but not the others: http://grab.by/36FP

- The main feed seems a little too text-heavy to me. Have you considered putting profile thumbnails next to questions and answers that appear in the feed, as you do for comments on question pages?

- Your buttons are awesome. As are your tooltips.