how/why? Facebook is how I connect with my friends and make plans to hang out with them in person later. Without it I wouldn't talk to a lot of people, online or offline.
In exactly the same way that heroin substitutes for many healthier goal-seeking behaviors in the junkie, Facebook seems to give many of my former friends a social fix, such that they seek other forms of interaction much less now. That includes telephone calls, nights out at the pub / dinner / billiards, and especially one-on-one conversations.