Spamming won't work. You'll get people clicking the link to have a look, but hardly any of them will join and the ones who do will be unlikely to end up regular members.
The only way of successfully starting a forum as a "rival" to wp is if you already have plenty of members ready, ie you have a lot of friends from wp who would be prepared to post on YOUR site instead.
Otherwise, why would they bother going to a site with hardly any members and nothing new to offer?