Quick run down of my reactions...<p>Valve: This makes the least sense to me. You shouldn't enter a highly competitive market (like games) as a side project.<p>Twitter: I don't think all that many people care about the real time web. But even if they did the question becomes "Why couldn't Google create a search engine that crawls Twitter every few minutes?" They don't need to own Twitter to make a real time search product from Twitter's data (which is all public after all)<p>StumbleUpon, Last.fm & FriendFeed: In all these cases the author assumes there's some value in knowing what the users of these services find interesting. But the truth about all of them is they have a very small niche audience. Meaning if Google used them they'd be skewing their results to a small audience while possibly alientating everyone else.