Like it or not, Google seems to be the most-logical buyer. One can argue that Google could write an app that replicates Twitter.
But that's only part of the value Twitter, as opposed to any other micro-blogging and real-time search app anybody might produce.
Twitter has gone viral. It has mindshare. People like it and use it. Sometimes that trumps every other consideration.
A sale to Google alleviates the need to "find a business model." Google already has one.
And if one assumes Twitter always has been an acquisition candidate (for most firms the "going public" exit does not presently exist), then Google simply is the most-logical buyer. The value Twitter provides is congruent with the value other Google tools provide.