There are many things you can do to grow a forum. one is viral marketing. as an example. I love rugby, and I have a small rugby forum (about 300 active members & 400 posts a day) We delete members who do not post, or who do not reply to emails asking if they want to keep the account open to allow them access to stop a troll attack.
Anyhow recently we had something called the 6 Nations rugby tournament. It is held between Wales, Scotland, Ireland France & Italy. During the Wales v England game one of our players dumped a top England up and coming player big time. I made this image and placed it on my site the same day.
On Feb 7th, we had a great day. We had more uniques that day than the whole of the previous month (we get about 80,000 uniques a month).
Viral marketing is the way ahead, I had this image sent to me by email by dozens of people who didn't realise that it was me who created it. It went around the world, was featured in our National newspaper, Daily nationals like the times, on TV, and in rugby mags. And although I had one eye on this working, I did it because it was funny. I also spoofed the cover of the film 'the Italian job' when we beat Italy, and the final one was a spoof on a BT advert where I removed the BT line 'It's good to talk' And had our coach on the telephone to all the other coaches of teams we had beaten, and the headline
'It's good to gloat' .
I run scumv for fun, and as such I hold it back from getting big as I don't want to lose it. But if you do this right, especially with a teen site, then you can make a site big. One thing though, don't look so much at the membership, look at the posts per day, it is far better as an indication of popularity.
<note> I am leaching the image from my server, if it is not acceptable feel free to remove it >
