[The Geek]

If your forum was essentially a massive shoutbox then yes, Twitter will kill it (and it would deserve to kill it). Forums are discussion hubs. Twitter and Facebook are not.

Saying that, there are forums that FB, Twitter and 1,001 other sites will rightfully kill off. This isn't because FB or Twitter are in competition with forum owners rather some forum owners being in competition with FB and Twitter.

Visitors time and attention spans are finite. The number of sites vying for visitors attention is growing. The more time visitors spend on other sites the less time they have for yours.

If your site sucks and doesn't offer any interesting focus, discussion or community then I think you're probably screwed. Gone are the days where you could substitute quality content with a points system to buy members "beer.gif" or an arcade leaderboard as a method to hold a visitors attention span.

If visitors want little games (distractions) then FB or several other billions of sites will grab your visitors attention far better than mods will. Forums relying on irrelevant whistles and bells, Forums not playing to their USP (community & discussion centred around a niche subject) are doomed.

If you are a community of Widget lovers, then focus on content and discussions about widgets. Everything else just works against you.

*before I get flamed, I am sure the arcade works on many sites where it is relevant. Maybe even some where it isn't. But FWIW, I don't see many big boards ( > 1m posts) with leaderboards and points systems anymore. I see big boards dedicated to discussions.