Just had a look at Webmaster Tools and it was showing 5445 links from a forum I made for a local body corporate. This site has maybe 15 members and 100 pages. I put a link in the footer. When you examine the links they all seem to be of profile pages of invalid members. Theres no info or backlink. And then there is this message "The member you specified is either invalid or doesn't exist. It looks like the page is created when a bot tries to register, not when the registration is completed. Therefore growing in number rapidly. What I'm thinking is that somehow a false positive is being sent home and the pages are being pinged to get the links indexed. I don't really rate sitewide links on forums much, the main reason I'm telling you this is for anyone running one, or anyone with a too good to be true success rate on MyBB forum lists.