There's definitely a cutthroat competition going on between Facebook and Snapchat, but even more generally I think it's that any app which is primarily designed for communication will eventually feel pressure from their users to expand and branch out until it converges on a certain set of fundamental features. Every app claims to be different and to change the way that people communicate, but really, the people themselves and the fundamental urges that drive communication stay the same.

There's a long history of specialized communication apps gradually expanding as time goes on:

- At first, Snapchat was all about disappearing photos sent directly to a single person. Then they added 24 hour long stories which can be seen by multiple people. Then they added one on one text chat. Now they're adding group chat.

- Instagram was originally for small, square resolution photos with an artsy filter. Then they bumped the photo resolution and allowed non-square photos. Then they added videos. Then they added a text DM system. Then they added group chats.

- First Twitter was for short, public, text only messages. Then they added photos and videos. Then they added private messages.

etc, etc... (my ordering on some of those might be wrong, but you get the idea)

It almost seems inevitable that whatever specific thing that differentiates a communication platform will gradually be eroded away until it resembles all the others.