A considerable buy in? A full acquisition?
And, assuming a full acquisition... what would be the gain?
Google has a bad story with attempts at social media, apart from YouTube. (Bought Orkut, killed it, tried Google Plus, went nowhere). Twitter is hard to make profitable without alienating the users with too many ads.
For Google, it would probably be an acquisition like YouTube. With the knowledge that it might never be profitable, but intended to get control over a significant asset. But sharing Google infrastructure and resources could probably bring down operating costs in the medium term.
We'll see.