I think it's essentially part of the fabric of the Internet today, and far less replaceable than something like Facebook
There are many examples of communication mediums getting abandoned suddenly and rapidly on the internet, just off the top of my head there's BBSes, ICQ & MSN Messenger. Even email to an extent, I get much less personal email than I used to and it seems to be much less common for young people to have their own email address.
What people use to talk on the internet seems to be an incredibly faddish thing.