That would depend on what you are trying to accomplish. If you are trying to get human visitors to read your post, see your signature, and then follow the link in your sig, then one post in a forum is definitely not going to get you anywhere. If you are trying to build links, then no. You will need more links that just one from a forum. I wouldn't recommend using forums to try to build backlinks, though. Although the backlinks will count, it is much more effective to acquire links from a wide variety of IP addresses and domains, as opposed to getting lots of links from only one website/source.