It's licensed under GPLv3 -- in my experience, that's an instant turnoff for most .NET (C#/F#/VB.NET) developers. If you changed the license to something like Apache 2.0, I think you'd get much more interest; not only new users, but new contributors too.
Most -- nearly all -- major open-source projects written in C# (or other .NET languages) are permissively licensed. Yes, that means people could just grab the code and use it in their own closed-source projects without giving back; but, as far as I've seen, most devs are happy to contribute back to the projects they use in whatever way they can.
[1] http://www.gnu.org/licenses/license-list.html#GPLIncompatibl...