i think we used to have a button for this.. seems like it would be worth an experiment adding it back, at least for a short time -- something really low key and not at all in your face.
Go for the Facebook Like button then
Or perhaps the sharing mod instead of the "Social Bookmark The Page" link at the bottom.
Also, Facebook Connect may actually be a good platform for here, as it will remove the need for new users to actually sign-up and participate. Instead they can simply click "Connect with Facebook" and then have thier account linked onto here so they can take part in the forum.
Creating a forum post specifically for these people highlighting the benefits of being a member of DC may actually bring a whole lot more people here.
Think of a pyramid, if one person hits 'Like' on a post, that gets posted to thier facebook profile, if they have 500 friends and 5 of those look at the link and also click like after they have connected with DC, they will also share the link with all their friends, if they all have 500 friends, thats another 25 members just from 1 member, but those additional 25 will each bring in 5 each, being 125, then another 5 each, and so on and so forth...this is all from 1 facebook like button.
Not to sound too much like a fanboi but Facebook is one of the biggest platforms for free advertising, as well as paid, and in fact, people sharing links on thier profile, is how things go viral. Personally, I think I have clicked the whole of 3 paid advertisements on Facebook, but I have clicked on thousands upon thousands of links that are shown on peoples profiles, simply because they said they liked it, and knowing I have roughly the same taste in content as these people, I clicked it seeing if it was of interest to me. This has introduced me to countless websites that i still visit each day, and has let me stumble accross an amazing amount of information accross my time online.
Forgot to mention, the Facebook Connect utility has saved me hours of time registering on websites that require you to sign-in to view posts and/or reply to things I enjoyed reading or needed more information about.