Facebook is a social utility that can easily be utilized to gather traffic to your site. Even if you don't get much traffic, it's good for link-building to, a key (as any competent internet marketer will tell you) to SEO.

As always, the first step is to create an account. Chances are good that you have an account already.Even if you don't use it, I would suggest creating a new one, for a couple of reasons. Firstly, you may want to use an account under your name in the near future. And if you're site is on, say, camping tents or nursing homes, then your friends will probably wonder why the heck you're joining weird groups. It's not hard to create an account at all either.

When you create your account, Edit your profile a little. Add some details about your life (or just make up some believeable facts about an imaginary person). This helps your account to look legitimate to other people, and to Facebook. It isn't wholly necessary, but it does give somewhat of a boost.

Next, you'll want to join some groups. If you look at the navigation bar on the left side of the browser window, you'll see an option titled 'groups'. Click on this, and it will pull up a page of your groups. You'll want to look around for groups related to your website. Join somewhere around 10, if you can find that many. You can do later more, but 10 is good for starters. You now have a couple of different options. You could post on the wall. This won't work so well (if you're aiming to get direct traffic) in larger groups, because their walls tend to move very quickly, and posts get pushed off the first page. Of course, there are those gems with tons of people that don't use their walls so much, so be on the lookout for those.

As for the number of members in a group, it wholly depends on the group. Some groups with 100 or less will have tons of activity, while some with over 1,000 members will have little or no activity. There isn't much of a limit or a minimum; it depends on how involved a group is. So try to find groups with high involvement level.

Another thing you should do is start your own group about your topic. This ensures there's a group on exactly your niche, and you might actually get people to join it (don't expect it, unless you go around and ask a bunch of people to be your friends). On your group, put a paragraph or two about your topic, and commend discussion on the topic.

One last thing that you can do is look around in groups  and ask a bunch of people–preferably those who seem to be interested in the same things as you site-wise–to be your friends. This gets some internal linking going around your profile page.