To promote a similar forum site in your first post is like showing up uninvited to a party at a neighbor's house and inviting all his guests over to your place for a similar good time.
There's nothing wrong with letting us all in on another good site, but the recommendation has more value once we know who's giving it and if they are in any way affiliated with the site.
If one is affiliated with the site, there's nothing wrong with including a link as your homepage in your user profile. The folks around here will usually check out the homepages of noobs who have something interesting to say in the threads.
[disclaimer: my homepage link is just a joke] :p
If you have a really, really, cool homepage and are bold as brass you could even put a link to your own website in your sig. But it better be worthwhile or there might be some criticism. If it's very well done, there will be much respect and admiration.
