The OpenSSL library is mostly used on the server side and is currently being used by many web servers. In fact, as of 2014, almost two thirds of all web servers on the internet make use of OpenSSL.
You are absolutely right about the libraries that you've mentioned. None of the major browsers make use of OpenSSL. Most of the libraries mentioned in the question are protected by under copyright by their respective vendors. In fact, the SChannel library is not even Open Source. youYou can find a list of these libraries here.