I wouldn't say Facebook has a huge problem here - users spend much more time on Facebook than on Google (by design) and even if they don't intend to do anything other than "use Facebook", the ads are effective as a result of their high exposure.

That said, however, a very large group of users are there to play social games. With social game companies competing for ad space (for these often highly-profitable users), they're driving up the price of ads and making it less economical for other industries to advertise on Facebook.