A "corporation" is any time two or three people gather together in something's name. It's any kind of, well, corporate entity, a single thing comprised of multiple people. A school is a corporation, a town is a corporation (seriously, many municipalities are legally incorporated), a marriage is a very limited corporation, and a business is also a corporation. So, yes, Wikipedia is a corporation, and it should be proud of the fact it can keep so many people working towards a common goal.