The one major option I can think of (if your haven’t done it already) would be to configure the facebook login, it’s pretty easy to do and it will take for your users 5 secondes to create an account. The facebook logins will import the profile pic too.
The Quick Message plugin is also interesting for big facebook users, it (kinda) reproduces facebook messenger. They can send MP pretty easily. But it’s more an idea for “after they are lured in your community” proposition.