Numerous people that hang around the webmaster world forum have claim that Google is currently "sniffing around" to see if Twitter is up for purchase.
After Orkut has miserably failed (you might have not even heard about it being Google's social network) - they now aim at the world 2nd largest social network and one of the top 5 trafficked sites in the world. This makes perfect sense to me and I concur it would be a wise move upon Google's business development team.
The problem with such move is that Google would integrate social signals from Twitter into the search results, i.e: twits mentioning websites will be counted as links and improve a website's visibility. That is currently being practiced already by Google to some extent, but when they gather the full data from Twitter, results will be changed dramatically. Millions of website owners who have not been focusing on Social media effort will have their assets harmed, at least to some extent, and even if they were doing some Facebook promotions - it will not be credited as much as twitter. That could and should cause an immigration from the love-able and much better Facebook towards dull, uninteresting, Twitter.