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Forum Moderators: rogerd
Thanks in advance for your input!
As for instant messaging, Jabber is an open standard that has a large following already.
Honestly, though, your best bet is to just give your users a way to connect to each other with their existing IM solutions. You'll have a hard time convincing and then educating your users to download, install, and configure software to use for your service.
Most community software offers both private messaging (which is personal, but less instantaneous than an IM) and the ability for users to register their existing IM usernames with the community. This usually involves a field they fill out in their profile which leaves a link by their name whenever they communicate with other users or when other users view their profile. It'll be easier for you to convince your users to fill out this profile field than it will be to get them to sign up for a new service.
[edited by: rogerd at 6:58 pm (utc) on Dec. 4, 2007]