Are there social Q&A cms?

Question

CMS

We need a question and answer site with the following features:
1) write private messages (the ability to correspond with them on the site)
2) online notifications on the site about new answers, comments, private messages
It is desirable, but not critical, to be able to:
3) log in through social. networks
4) ask questions to the user
5) follow / follow the user
I tried to use question2answer, everyone liked it, but the lack of the necessary first two points forces me to look for a replacement.

Answer 1

q2a has notifications and I think there is a plugin for private messages.