SocialEngine and Jcow are both PHP, you can see that simply by typing their names into a search engine.
Though the programming language shouldn’t matter for you. Use whatever you’re comfortable with - if you know .net better than PHP, use that. Both are as capable as eachother.
While I’m here, a bit of advice. If you want to make it big, come up with an idea of your own. Those sites have already been made, and are both largely based on a website which preceded them - facebook. Their combined popularity and userbase is much less than facebook too.
Come up with something new, that way you’re not competing with websites worth billions of dollars. You’ll be competing against nothing and can possibly make it.