Indeed. For such filtering to work, a significant portion of your friends should be searching the entire network and not just filtering through friends. But this might just work out to an early-adopter/follower divide. Friend network filtering will work better for the followers. The early-adopters/leaders will continue to use other means of search and will feed the friend-network.
That assumes the same group of people will be the early adopters for everything, from recipes, to gardening tools, to what video card to get. If your peers don't happen to be knowledgeable about what you're seeking, then you're SOL.
Your best bet ends up being to simply include everybody, which improves your chances of finding an expert opinion, regardless of the obscurity of the query.
