Basic assumption: you trust your friend more than a stranger. A friend of your friend is probably more trustworthy than a random stranger as well (at least because your friend trusts her). For example, in the context of movies tastes, if all your 10 trusted cinema-friends trusts "mary" (that you don't know) about her movies tastes, can we predict that you will find her opinions about movies useful as well? I Dont think so.