I doubt this statement is true.
There are more colleges/universities teaching C++ rather then C as the first programming language. Then it should be more people knowing C++ rather than C.
There's knowing C++ and there's knowing C++. It's an incredibly compex language and continues to evolve for some reason making it less portable than it could be. So it's mainly used in silo'd environments.
C is portable and ubiqutous and sufficient for small/medium sized projects such as the typical open source stuff; thus more common.