Yeah, but where will those ads appear? I have absolutely no data on this, but it seems people are spending less and less of their web time on "the open web", and more of it on medium-to-huge websites, and those usually sell their own ads directly. Facebook will succeed at what you're suggesting only if their targeting proves so effective that medium-sized websites (say the big blogs and online magazines/news sites) can make more money running Facebook ads (after giving Facebook its share) than from staying independent.
They'll appear anywhere you see a site currently running adsense ads. There's tens of millions of sites running those ads, there is far from a shortage of publishers for a new advertising network.
Yeah, people are crying out for an alternative to adsense but can't get anything near it due to the amount of information Google has to target ads and the inventory size. Facebook is well placed to offer as good returns with even better targeting. Just look at the amount of targeting options you get on Facebook now, I'd take those any day to what adsense gives.
