If it's
www.persia-cms.com , then phpBB SEO is already giving you at
least 181 backlinks
So for sure, being active in a community forum being itself well indexed is a good thing.
The principle to follow when willing to increase number of backlinks using your community activities, is to do it honestly.
Not really to keep morality safe, even though it can count, but just because it's more of a pleasure if you really like to be part of the community you're posting in. Your post will be more interesting and you'll last longer doing it.
I mean a link is a link for a bot, but we shall never forget that bots are not our target, users are. And they will read what you write in other forums. That's why I find spammers so stupid, because, at least for me, there is just no way I'll follow one of the dirty spam message links, just because it's too obvious it's crap, while when I fell on someone telling interesting things I often visit his web site.
So number one to me is to find a community reaching one of your real center of interest. Then, of course better if your site is related to this center of interest of yours. Backlinks coming from website related to yours are better than the one coming from sites covering other topics.
Another interesting thing is when posting in a related forum, you can from time to time post a deep link to your site, when it makes sense.
Google does use the ratio (number of link to home page) / (number of deep links) in the PageRank calculus, because they tell the Bot the web site does have interesting content as people are referring to it directly).
So these are gold, but you'd better make sens posting them as in most forum now, they check for spammers, which leads us back to first point : choose as community you like and can be useful to.
Then, better if the web site is SEOed, but it's not the only matter to take into consideration.
For example
www.phpbb.com is not SEOed at all, but it's PageRank 9 make it so interesting to bots they do manage to spider it :
1 570 000 indexed pages, only three are out of the omitted results, but almost all of them
are cached.
We can see from this example the Google ability to spider URLs with SID and un rewritten, but you'd better have a PR 9 to obtain this result, and it's a great pity 99.99% of page are left in the omitted results, even if this does not mean these pages will not show up in Google results. It's just that they are less considered than they should.
The ending point here, is Google is catching backlinks from phpBB.com. But this would not be the case with a PageRank under at least 5 I'd say.
Actually, the mxBB website only had few pages indexed and cached (like 200) before it started using the mx Google sitemaps, was with PR 7, so yes SID can be spidered, but if PR 7 isn't really enough ...
So to answer I'd say, first choose a community you like, then check if they are indexed
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