"Bit invasive, i thought. Why should i want to know?"
'Cos TripAdvisor is paying them, perhaps. Fact that they are known to you might sway you, if you "value" the choices of your "friends".
I just hope they don't give the dates of their holidays online, and leave themselves open to burglars though.
I suppose it can work, if you have a small number of friends and you also think whatever service / site is reputable. It can, of course, give a negative view of the company (and / or of FaceBook ) for using such methods, if you consider it a breach of privacy.
Wasn't it FaceBook which once divulged items being bought by friends (and thus spoiling a number of birthday "surprises") ?
Lucky for me any mail from FaceBook (or supposedly from FaceBook) gets thrown in Spam by GMail...