Errrr - I never advocate participating in forums strictly for links - instead, try to make a short list of forums where you can actually either learn something, or build true relationships (These can be local sites, real estate sites, niche specific sites for something you are interested in etc) - there needs to be a lot more in it for you than a link if you are going to dedicate the time to make contributing to any forum worth it.
A good real life example would be the local forum we run for our city Nanaimo, British Columbia.
http://www.nanaimoinformation.com/forum/
Here a local Realtor could get in front of a very relevant and local audience, and perhaps even get more than just business out of it (Friendships, or maybe they are an outdoors buff, or a photography fan etc) there are lots of things that make our local forum great.
Now our city is not huge - but we have over 3% of the total population as members at the forum, and if you were to figure out how many of our population are actually home buyers, the % would go up because obviously our stats count children and the very elderly, which were not be home buyers AND would likely not be members at the forum.
The Realtor could have a signature link, and would get link juice from it BUT this is probably the least important reason for that Realtor to be there.