If the forums have dedicated marketing/classifieds sections then using them is an acceptable practice in itself (especially if they nofollow the links or block crawlers from the marketing sections, which many do these days).
But copying the same ad to each and every forum is generally frowned upon (I would not like that in my own forums although I know it happens from time to time).
These kinds of advertising sections work best for actual, productive, active, long-term forum members. The other members of the community know your history and are more likely to trust you.
As far as "link building" goes, that is a path fraught with peril. Yes, everyone in the SEO world says you need to build links (opinions vary on how many, how, and for how long, etc.). Nonetheless, link building leads to a huge majority of the penalties and downgrades that Websites suffer in the search indexes.
It's better to write about your business, products, and services on a public blog that is quickly indexed by the search engines so that people find your site organically. By building up visitors to your site gradually over time you earn their trust and from their feedback you learn how to publish the kinds of articles that they will share for you.
You can compensate for a lack of organic traffic through pay-per-click advertising (although if you need a lot of traffic right away it's better to hire a professional to help you manage your campaigns than try to do it all yourself).
There are tons of ideas for attracting links published across the Web (most of them very bad ideas). You should first read through the Google and Bing Webmaster guidelines so that you understand what is basically permitted; then begin experimenting on a small scale with simple ideas for attracting links and gradually improve your knowledge and skill.
Rapid link acquisition is your worst enemy. Don't let anxiety about your competitors' "progress" drive your business plan.
Edited by Michael Martinez, 21 March 2013 - 09:52 AM.