Once registered on facebook, the key to effective networking is finding and making "friends."
Now, I haven't done this as rabidly as others have. In fact, I think there is an advantage to building a facebook network slowly over time. How can you effectively interact with people if they are only a name? And I don't friend everyone who sends me a request either. 98% of the time I do, but I always look at their profile. If there is no info there, or a lot of objectionable content, or if they seem like just a spammer, I ignore their request.
How to find new friends on facebook.
Watch the friend suggestions facebook gives you. I don't friend everyone that is suggested. (For example, I'm not going to friend my friend's cousin. Or the person I went to school with, but have no recollection of.) But I have added a number of friends that way.
Join facebook groups. Read the message boards, and add friends who say things that interest you. I'm a member of the ACFW (American Christian Fiction Writers) and I joined their facebook group as well. Immediately after joining, I got several friend requests, and have since sent more to other members of the group.
Steal friends. OK, I'm guilty of going though other people's friends list and friending their friends. Especially writers.
Look for links on message boards, blogs, and email signatures. If you interact with someone in some way online, why not interact on facebook too? And why not put a link (or at least mention you're on facebook) in your signatures?
Search. I tried this last week. I searched through the Buffalo network looking for writers, and ended up adding a few local authors I didn't know about.
Now, I've learned not to take it personally if people don't respond to a friend request. Some people simply don't want to network on facebook. They use it to link up with family and friends. Agents, for example, are unlikely to friend writers. And that is certainly understandable. And sometimes people clean out their friend's list, and I try not to take that personally either.
But once you do make friends, try to interact in meaningful ways. I usually send a private message or "write on the walls" of new friends. I often don't do this if the person I'm friending has 1000 or more friends. When their friend's list is that long, I worry about just being "one more thing they have to deal with."
Which is perhaps why the best networking is often accomplished among other aspiring writers: Make friends at the same level, and grow up together.