I experienced the same inability to authenticate in FB and Twitter yesterday. Luckily, after several attempts this morning I was able to get both accounts logged in. I ended up turning off Wi-Fi and 4G, and I was successful logging while on 3G. I have no idea if that made any difference, but it's still working now.
Anyone know about a supposed Social Hub widget? I watched a "first look" video on YouTube and there was a demo of a Social Hub widget on the Epic 4G Touch which even featured a scrolling content window. Anyone, anyone?