Ok, as you may know, vBulletin 4.0.3 incorporates Facebook Connect login, I thought I would go into some detail as to why we have incorporate such a feature when we still have a number of outstanding bugs in vBulletin 4, as well as why we are planning much more detailed integration in 4.0.4
We have been testing with integrating forums with Facebook for sometime to see if it has merit, how best to incorporate it and what forum owners and publishers can obtain from integrating with Facebook.

Why Facebook?
Well, the immediate ability to expand your potential userbase by 400 million (and growing). Facebook may not be the single long term solution when it comes to collating/storing information about people online, everyone knows that the history with sixdegrees/friendster/myspace et. al. and Facebook could certainly fall into the same category, but it is certainly looking like it will be the single largest source of users for the foreseeable future.

How can it improve the reach of my site?
We have done some testing integrating facebook logins and publishing, with some great success!
Our referral traffic from facebook on our forums where we have integrated login/publishing to facebook and a well managed fan page has seen a 100x increase.
It enables you to reach not only your existing users, but also other potential users. If you have a site about cat lovers, chances are a lot of users will have friends in their network will like cats too. We naturally gravitate towards individuals that share similar interests

Isn't Facebook a competing entity?
Well it may compete for your users time, but the fact is, your users are most likely going to be there anyway, you may as well be there with them, and make sure they click off Facebook to your site.

Outside of the features that we have offered, some suggestions for you.
Get a presence on Facebook. Build a fan page for your site, let your users know about it, and perhaps get a trusted user to update it for you. Put some content on the page, but also use it to build dynamic conversations with your users. Facebook referral traffic on some of our forums as mentioned has increased 100 fold since we have embarked on maintaining an active presence there.

Invigorate them with interesting content. Consumption of content now days is incredibly sporadic. If you have a site on garden care, and you come across a cool video on youtube, put it up on your facebook page. Doesn't matter if its not on your site, users are trending towards consuming more content that is "instantly gratifying" as opposed to reading through a 500 page thread on your forum. Use it to help build your brand.

To help you leverage this, we have "fast-tracked" the development of our facebook connect plugin, and will be bringing more features to it soon. You can download the latest version of vBulletin with facebook connect here: http://members.vbulletin.com/
Instructions on how how to integrate facebook can be found here