
Recently I read The Fall of Advertising and the Rise of PR which I think applies more strongly online where adverts are completely ignorable, but an audience will flock to something interesting.
Compare the average banner ad (which almost no-one clicks on)...
Unless you can sell your product at a literal glance (snorgtees...), they just don't work.
To the Where I've Been Facebook app, that's useful to me (and also likely to sell several million dollars of air tickets at some point). And, therefore, incredibly popular.