There are shares outstanding, generally they are 'worth' the same, but with some differences maybe due to voting which affects price a little ...

Most of the companies stock are not 'floating' meaning, they are not out there on the market. So Zuck's shares are not.

The 'share price' is usually just the price of the last transaction of buy/sell on the stock.

If Zuck sold all his shares at once, he mighn't be able to get that price. But he wouldn't anyhow.

There's nothing nefarious about 'others contributing to his wealth' - people are generally buying the floating shares given what they think it's worth.

But here's a trick: if a company has a 'high profile' - often it's shares have some premium simply because there are more people looking at it. Supply and Demand.