There have a solution for making a MMo game using Unity,its free and opensource.
It have a stablish network(include client's network write by c# and server's network write by c++[libevent] ).
the website: https://github.com/ketoo/NoahGameFrame
features:
1: it is easy to use interface oriented design minimise the effort;
2: extensible plugin framework makes getting your application running is quick and simple;
3: clean, uncluttered design, stable engine used in several commercial products;
4: using the actor model has very high performance(by theron);
5: based on the event-driven and attribute-driver can make business more clearly and easy to maintenance;
6: based on the standard c + + development, cross-platform support;
7: with existing c++, c# game client for rapid development;
if u r interest to use this frame, pelased contact the author without hesitation.