Last week, Second Life's official fan page on Facebook gained over 200,000 fans, according to PageData. By default, someone who "Likes" a Facebook page receives updates from it, and since 50% of Facebook users log into the social network every day, we're talking roughly 100,000+ folks who are seeing SL content this way on any given day. Less than four months ago, the Facebook page only had 160K fans, and despite reports that some Facebook users with profiles named after their avatars have been banned, it's kept growing.
Second Life's monthly active userbase is about 1 million, so it's a safe guess that roughly 20% of that userbase is connected to this Facebook page. However, keep in mind only about 400K users are heavily engaged with Second Life on a monthly basis (i.e. logging in for 4 hours or more), and my guess is much or most of these 400K are hitting this Facebook page. Whatever the actual figure, it's a significant percent. And while Facebook isn't necessarily the best social media platform for sharing all SL content, it's probably the largest and most active. The official Second Life Twitter account, for instance, only has 27,000+ followers, and while Plurk has a very active sub-community of SLers, that social network's developers tell me they get only about "several thousand daily active Second Life users" on that system. So if you're looking to promote SL content and activity in social media, look to SL's Facebook page.
Another interesting point: As Second Life user hours continue to decline, Second Life's Facebook page continues to grow. So as I've noted before, the fact that people are logging into SL less doesn't necessarily mean they're engaging less with Second Life overall. Instead, what's likely is this: People are logging into SL less because they're engaging with SL through Facebook and other social media channels more.