I believe this is a failure of imagination. Facebook Credits will be their Gmail, the second act which shows that they are something special. Blippy, Groupon, and others are all in the general area of "social purchasing" but FB will simply dominate this when they enter the game. In a down economy, if you can get a discount by simply sharing or liking, you will. And advertisers can micro target not just demographics, but influencers within those demographics, people who are disproportionately effective in passing on memes -- and buy messages -- to their friends.

(Parenthetically, this also means people will visit Facebook for product search, buoying the overall commercial intent of the site. And an endorsement by a friend of something carries even more weight than a good Amazon reviews.)