Below is a series of screenshots from within the Facebook iOS app that allows users to match their Facebook friends against their iPhone Contacts: "Find Friends on Facebook: Choose contacts on your phone to add as friends on Facebook." It's a basic concept - but it's tremendously powerful since your mobile contact list is really your tightest, most significant network.... and those users are surely also Facebook users. Of course Apple performs the matching by uploading contacts from the device and then sorting them on Facebook's servers. You may remember this practice was critiqued publicly - but Facebook is very clear about how they are using the contacts.
It is also worth noting that Facebook is clearly aware of the potential growth here (in one click I can add 1,109 new friends!) but wants to balance some quality control: "Please send invites only to friends who will be glad to get them."