Okay, I'm sure there are a lot of us that has plenty of experience developing c++/opengl/objective C on the iPhone, java development on android, python games, etc (any client side stuff) while having little to no experience on web-based development.

So what skillset should one learn in order to be able to work on web projects, say, to make a facebook clone (I kid), or maybe a startup that specializes on connecting random fashionistas with pics etc.

I actualy do have some experience with C#/VB.net back-end development a while back, but as part of a team, I had a lot of support from the senior devs. Is C# considered a decent web development language?