I run a popular forum and we have certainly felt the effects of competing forums, Facebook, blogs, etc. I've run my forum for over a decade and there are more distractions now then ever, wether it be FB, Twitter, blogs, iphone games.
People need to have a motive or incentive to post on your forum. More importantly they need to crave it and not be able to live without it, and that craving has to be strong enough that they choose your site over every other distraction with their finite time. I love twitter and I'm able to use it in such a way that benefits my life. But after having our baby I've barely used it the past 5 months. I don't feel any differently about it, it's just that I can live without it. So you need to make your forum a place where they can get things they can nowhere else, they crave it, and I think this last thing is very important... it's super easy to do.
An example of this last point is we have a photo gallery, with tens of thousands of photos, but people are using it less and less because it's much easier to post on Facebook or Picasa. Consequently we have other features that are our bread and butter but people are using that less and less because of their gallery frustrations. You need to identify these problem spots and fix them, which is tough because people have developed extremely high expectations of ease of use because of the aforementioned services.
One thing to look at is how many people are visiting versus posting. It's possible people are visiting, but are not finding reasons to participate. Stagnation is a killer. People want to participate in active conversations. Find ways to get conversations going. If you have a facebook group post things there trying to get conversations moving onto your forum. Have contest where people find the best topic of the year or ever. Best question contest, best answer contest.
Another thing we have had to deal with are posters who post on every thread, where their comments make up half the thread, and others don't want to post because they don't want their threads hijacked by these over zealous posters. We've asked a number of people to tone things down and even has gone as far as creating groups with various levels of flood control to sandbox these people in.