Instant messaging is an example of something that used to work, and now doesn't. It used to be that everyone was basically on AIM, which functioned as an open enough network. Status information worked pretty well - you could fairly easily tell if someone was online, and so people could respond.

And now - we have dozens of clients, such that there's no way of knowing which subset your contacts use. With the prevalence of phones, it's hard to tell if someone is available or not, so you just send messages in the ether, and get surprised if anyone responds.

And the worst part is, there's no obvious fix, since any new system will just add to the fragmentation.