Web of trust
It's all very well to say that (were I actually to HAVE a Facebook account), I could search for, oh, I dunno, say a new tablet computer and weight the results by the ones that FB friends and FBFs of FBFs etc. "Like". I can see how that would be a fun and geeky sort of thing to work out, if you've got terabytes of linked Likes.
However, it's much more likely that I'm going to think of an Actual Friend who kno more than 0 about tablets, and I'm going to give him or her a call and say "What do you think of the new Applung Nexoid 19.3, eh?".
I don't necessarily trust the judgement of every one of my friends, and certainly not equally across all of them with respect to any particular issue. That social graph is going to need its edges weighting carefully if the search results are going to be anything but dross.