My web app needs a private messaging system just like Facebook Messages or Twitter DM.
I found a bunch of stuff that does real-time instant messaging, but they don't store the messages like a typical PM system does.
Plenty of sites/apps has a PM feature, and StackOverflow itself has plenty of questions about designing such a system, so implementing this myself seems like reinventing a huge wheel.
So is there such a service/platform that can be plugged into my web app? Ideally, it should be a hosted platform so I wouldn't have to deal with storing stuff myself.