Just a suggestion -- sewing buddies (whipstitch fabrics does this) with contests, or just regular contests. Free stuff and prizes are awesome. Linky parties are great too! Advertise the heck out of it (use the database of sewdaily subscribers). Let's face it -- use what you have, and bribe, bribe, bribe with free stuff, even if it's small, like a charm pack. Design contests so that the more you share the site with others, the more likely it is that you'll win something.
Also, online sewcial hours, where we post/share projects, or work on a single project at the same time. I want a date with my computer AND my sewing machine!
I love the idea of an online sewing community -- let's make this happen! Let me know what I need to do to help!
Let's do it! Drop me a line at firstname.lastname@example.org and we will discuss! Love your enthusiasm!