One question I have seen asked quite a bit on forums is about how effective forum links are for Search Engine Optimisation.
The short answer to this question is that such links serve just about zero use with regard to SEO and that if SEO is what you are focused on then you may as well not waste your time creating any forum links.
The longer answer is that SEO isn't the only way to bring visitors to your web site.The main reason that search engines take links to a web page into account at all in their search results is that the links themselves have value for two reasons. The first reason that a link has value is where real people use it to get from one page to the other. The other way in which links have value is specific to those added by someone other than a person connected with the page it links to in that where other people create a link to a web page they are indicating that in their opinion that page has valuable content that is worth reading.
Since the first of these reasons as to why links have value is the only one that applies to links that you place in forums, you need to consider under what circumstances you might be able to get real people to follow your link and find your site. The main reason that people follow links from one web page to another is because they think that the page they are going to will contain more information that is relevant to what they were on the first page to find out about. That means that where your link is to a web page that contains related information and the link makes it clear what it links to then you wil have real people follow that link. So if you pick fora that are on topics related to what your web pages are about and post links back to your site on those fora then some of the people reading your posts will follow the link either to find out more on the topic or to find out more about you. In order to get this interaction though you have to be making relevant posts to the forum and actually contributing to the discussion. In most cases that will be a requirement for being allowed to post links on the forum at all since most disallow self promotion and so your links will need to be in your signature (where the forum allows self promotion links in signatures) or will need to be so relevant to the topic discussed in the post that other people would post a link to your page as their way of contributing to the discussion.
Given the way in which forum links have value it doesn't really matter whether the links are flagged as nofollow to stop search engines from following them or even hidden from the search engines completely since forum links are targetted at real people and not at search engines.
One forum I post to regularly (almost 11,000 posts at the time of writing this) used to allow search engines to follow signature links for those with more than a specified number of posts but more recently decided due to the volume of spam posts by people trying to obtain the necessary number of posts that they would just hide all the signatures from the search engines completely. their making that change meant that the search engines suddenly had about 10,000 fewer links to my site to consider in working out where to rank those pages I had linked to in my signature. Not one of those pages had its placement in the search results change in any noticable way when those 10,000 links disappeared from what the search engines could see. That is why I can state clearly that forum links have no noticable effect on SEO.
Looking at the number of real people following those links to find my pages both before and after the change shows that because my posts are on a forum relevant to what my pages are about that I get a significant number of people who actually follow those links to my sites and that too was unaffected by those links being hidden from the search engines.
As for how relevant links need to be in order to have a value on a forum (or anywhere else for that matter), I occassionally see posts that ask questions where I have already written a web page where I give a detailed answer (looking for commonly asked questions is a good way to find topics to write about). Often when reading through the responses that have been given so far to see whether everything I have to say on the topic has already been covered (since there is no point whatever in repeating what has already been said) I often find that someone else has already posted a link to my page. That in itself shows how relevant the material on my page is to the discussion which means that I could probably have got away with posting a link to the page without getting banned for self-promotion however as someone else has already been considerate enough to post the link I don't run into the self promotion issue.
This article written by Stephen Chapman, Felgall Pty Ltd.