I couldn't tell you how much credit if any google would count it for. I do know in my google webmaster tools account where I can check my backlinks I do see a backlink from my twitter page. I don't think the ones at twitter that get the shortened address will help much like tinyurl and bitly type of addresses. And usually you can't get very good anchor text for the links at twitter. Facebook I don't post links on, I only use it as a personal page. Seems like google does count some nofollow link sites as backlinks and others it don't. But how much it of that or how good it might help you who knows. I like a good combination of backlinks from a variety of different sites.
our pages on twitter might show on serp, however that doesnt mean we are getting backlink credit from those nofollow links. may be matt cutts could clarify this