Leanstack was on Bootstrap 2. Chose this because it is wildly popular, so it’s active, has been used a lot in production, and has a ton of features. Anything you need to do from a UI perspective, there’s likely a plugin for it already part of the library. Haven’t tried the others, but we're happy with BS.
For StackShare, we upgraded to Bootstrap 3. I don’t like that they changed the name of columns, essentially breaking the grid layout for Bootstrap 2 and below, so that was a real pain to update. I hope they don’t do that again. Once we have more bandwidth, we’re totally going to decouple our markup from Bootstrap.