All s-corp and c-corp corporations have stock. Valve is almost certainly an s-corp, because managing an LLC with that many employees just doesn't make sense, he'd want the structure of an s-corp. Watch The difference between a private and public corporation isn't whether they have shares, it's whether those shares are traded on public stock markets. Microsoft wouldn't be able to buy shares through a brokerage, they'd need to approach the share holders individually and make an offer to buy out their stake.

That's one reason why working for shares at a startup is so risky, it's really hard to sell private shares. If the company does an IPO (Initial Public Offering of shares, as in they're making some of the private shares available for public purchase), then you can cash out big, but until then, you have mostly worthless stock. Most of the shares are either retained by the owner, or by investing groups.

Watch Shark Tank sometime, when they say X% ownership in the company, they mean share ownership. The sharks want to get the company to be bought out or go public so they can cash out, otherwise they'd probably prefer a percentage of net income.

The definition of "hostile takeover" is getting 51% w/o a formal agreement. So if MS approached everyone independently and got 51%, it's a hostile takeover. And they don't need 51% control through their own entity, they can pay independent people to do so.

I think that's unlikely to happen, but it's possible.