If so, what are some contest you do.
For example, we have a thread called "last one to post wins (2012)". The thread last all year and the winner actually does get a prize. I think we gave away a flat screen TV last year. The year before it we gave away an Amazon Kindle. And I'm fairly sure we gave away a Mac Mini once too.
These type of contest are long term and not short term.
The short term things are not even contest, but rather group involvement. Such as Secret Santa for the community where members exchange gifts.
Another short term is raising money for charities. If X happens the site will donate Y on behalf of everyone.
We by no means depend on such things. They're mostly fun things to help members feel involved physically and not just digitally / virtually.