Their real mission is something like "exploit cloud computing".
And to the extent that it's about "the social graph", it's about making money off the social graph. There's nothing inherently wrong with that, but as Google becomes increasingly involved with owning, controlling, and/or providing content, as opposed to just "organizing" it (indexing the web, organizing email, etc.), their mission statement becomes more PR fluff than truth.
With Search and Gmail, their ability to obtain user data at least hinged upon Google's superior ability to index web and email information and make it "universally accessible and useful." Obtaining social graph data through an existing social games developer isn't like that at all.
And this particular case seems to very strategic for Google. It's believed (inside Google too) that Facebook could be the major thread to Google, if people start searching in Facebook first and go to Google only as a second option. Thus, attacking Facebook directly is very strategic move.
And it seems to fit nicely to Android strategy too.