On a side note - I'd take issue with your suggestion that the information on Facebook is largely useless to an advertiser.
There doesn't have to be any explicit "truth" to the action being measured (whether it's what a user happens to press Like on, or some abstract measure like their ratio of likes to logins), all that matters is that it's a measurable predicate of some other behavior that advertisers care about - like the language you're likely to respond to, what stage in your life you're at, your self image, what the rest of your peer group is into.
In many ways this a lot like a first date as well, in that the things your partner may embellish or overcompensate for might actually tell you quite a lot about what sort of person they really are.