I understand what the core functionality of Facebook involves, the fact that you must join does not make it "closed" at least by any reasonable definition that

I understand what the core functionality of Facebook involves, the fact that you must join does not make it "closed" at least by any reasonable definition that affects developers. Also the amount of control Microsoft has over developers and users is not comparable at all with a service like Facebook. You're talking about an OS that comes pre-installed in the computers most people use, they generally don't know how to keep it updated, much less remove it and install a different one. Anyone can join a new web service. Oh and btw part of the reason Google stays at the top (at least with regard to revenues) IS because they're at the top. Having an enormous inventory is a big part of the ads business.