The userbases are definitely comparable, even if the impact and potential revenue differs -- Orkut did not have the same reach in the U.S. as Twitter currently does. Also, AFAIK, Orkut was always a part of Google? I was thinking along the lines of, has Google shut down something that they paid billions to acquire? In terms of pure dollar amount, assuming that Twitter will go for at least a billion, only YouTube, DoubleClick, and Nest are in that neighborhood.

Too soon to make a verdict on Nest, but YT and DC seem to be the kind of acquisitions that Google paid a lot for, and from which Google advanced its bottom line and reach. FeedBurner ($100M), on the other hand...$100M is not nothing, but not as major as a Twitter acquisition would presumably be.

(AFAIK, Feedburner is still alive, but the APis have been deprecated a few years ago)

That number's not correct.