You could always do what some people do, and have you and your friends just spam links to your site and services everywhere they go. That seems to be allowed, acceptable and not at all comparable to telemarketers calling multiple times a day and using someone elses phone like it is part of their own business.
One option is to brand yourself by adding valuable content everywhere you go. Most places offer a place in your profile to link back to your website. If you become a respected source for your niche, people will naturally seek out your website.
Making connections with related niches is another option to consider. There are many websites that share the same niche but do not offer a forum or any means of commenting. You could try to partner up with these websites.
Paid options. If you have the money you could advertise your website through high traffic websites or search engines.
It's actually not as bad as you think. I run follower campaigns only.
I am averaging 32 cents per follower, at 5$ per day using a 60 cent follower bid. My wordpress site makes more than that, so it's not really a true loss. My forum doesn't make any money at all (no ads yet), but the goal of it is actually to drive site engagement.
I used to run a lower bid, of 40 cents. It cost about 19 cents per follower, just got less per day.
In that, that costs an average of 3 cents over a 7 day period to push people to the site from twitter, per site conversion.
One way to keep the costs down is to run a retargeting/look-a-like campaign.
Target existing twitter followers that don't currently follow you, or their look-a-likes, or people who are similar to your already existing users.
Do I make money on that? No. Definitely not. It's a losing proposition, however I'm not so concerned about today's money, and more so concerned about tomorrow's viral growth. So long as the site makes more money than it costs (including paying authors), I really don't care about profits at this point.
Honestly, I'm in this more for the fun of it than profits.
My site is techraptor.net. We have close to 300 articles a month. "SEO the hell out of it" really isn't an option, since I can't get mostly volunteer (they don't get paid a whole lot) authors to write SEO perfect articles. It's also a news site, and one problem with news sites is that it takes some time to google index anything. (We do ping to make it faster, and run sitemaps)
By the time Google indexes it, it's no longer a hot topic
We have some basic SEO checks in place (automated tool has a checklist for them to meet), but it's like herding cats sometimes.
Posting on other similar forums as someone who knows what he/she is talking about. It's about YOU not the website you're on. People are always looking for the better alternative, you just have to provide it. Your brand should be simple and easy to remember also.
Having a Facebook page etc. has done very little for us, if anything it has kept people away.
How old is your forum? Yes 15 is a small number but it's not 0 either. Really, 15 isn't a small number... it comes down to what you're posting/updating with. Not every post or article will fly to 300 replies.