I'm not entirely sure it's necessary. Any agnostic who wants to find forums to discuss the issue is likely to enter the terms 'agnostic' and 'forums':
http://www.google.co.uk/search?hl=en...c+forums&meta= http://uk.search.yahoo.com/search?fr...c%20forums
The community will grow about as fast as people want to come here and discuss it. Anyone who really wants a place to discuss agnosticism and related issues should struggle not to find this site.
That said, if you want more traffic the suggestions of videos on youtube etc aren't likely to hurt. You just need a good video about agnosticism to go there.
I have a set of forums and website on an unrelated topic where religious discussion is not allowed. If the opportunity arises I will certainly direct my community here, and I could possibly post a link on the links page. It's not that big a community though so not likely to give that much traffic, but evey little helps.