Not questionable at all- it's a gold mine for knowledge of value to advertisers. Every post, like, etc. on Facebook is spontaneously coming from a user. You don't have to infer what people love or hate based on browser and search history, you just let them tell you (and all their friends) what they love or hate. They're literally telling you what you should advertise to them, it doesn't get better than that for businesses driven by ad revenue (not a negative judgement, just an observation). Of course Google would want that data, and of course Facebook would want to prevent them from getting it.
The caveat being that on the level of the user it's more like (love, sort of like, guilted into saying you liked) vs (hate, feel kind of meh about, guilted into saying your hated).
The aggregation of those likes is more useful but at the individual user level it's a terrible signal. Especially since the entire social structure of the network is geared toward getting you to click like whether you actually like it or now.
