As someone who advertises on Facebook - I'd say that FB takes a lot of care to ensure that the ads it displays do not take away from the User Experience, both in terms of content relevancy and design (FB applies penalties on neewsfeed ads which have >20% text on the image). Further, since pricing is dynamic - crappy ads get expensive to run really fast. That's how this system is so much better than the constant spam you were used to seeing in Web 2.0.