I totally disabled mine on my forum..because people were soliciting other forum members for unwelcome business and then conversation would happen in PM's and not on the live forum.

I noticed once we did this our conversation on the live forum went up and also helped traffic. But I also thinks it depends on the subject matter of your forum as well, if that is for you to do that. In our case ...it was the right choice.

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www.digitalsignageforum.com
My Photo, Beadmaking Misc Art Stuff
More Niche

Ya, that might be a bit extreme for me but thats good that it worked for you.
Members keep asking me to install a chat feature but I haven't yet for that same reason, I know that would decrease posting significantly.
One thing you might think about is limiting PM storage for members down to like 20 or something, that way they still have the option to use it if its important but it will be too much of a pain in the ass to keep deleting messages and having it say "inbox full" for them to use it as a regular means of communication.

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Photoshop

White mike is true about his assumption.

Check your plugins and verify what hooks they maybe using.

maybe integrating a chat would be a good idea if people are using private messaging alot? They maybe using this as a means to communicate privately. Flash chat is very easily integrated into VB, and is free of and current SQL injections.

You could also put a time delay on how many private messages a person may send over a time, and how many at one time.