Not every medium lends itself to advertising, even if we’re talking about tends if not hundreds of millions of active users.
Sir Martin Sorrel insightfully points out:
The point is that Facebook is a social medium, not an advertising one, like search or display. It certainly is one of the most powerful, if not the most powerful branding medium. It is, however, a word of mouth or PR medium. You interrupt social conversations with commercial messages at your peril.
I think this is especially true with respect to Twitter. With all of the personal data Facebook has on its users, advertising there can be laser targeted. That’s not to say it’s a winning strategy, but there’s something there.