When I get on Google and search, typically there are a few ads keyed to my search. These ads are selling real products, often the exact product I'm looking for, from legitimate and well-established companies.

When I get on Facebook, my sidebar is filled with ads. I would estimate 50% of them are scams. About 25% are for Facebook games, which I am not interested in. The remaining 25% are legitimate ads which are rarely targeted toward something I want, but even when they are, I won't click on them because of the 50% of ads that are scams.