How can more people sell than are buying?

Short answer, they can't.

Doesn't someone have to buy the stock for them to be able to sell it?

This is actually the reason why the prices go down. Let's say Facebook's most recent trade was $38 and let's say there are a lot more people who want to sell than buy.

Someone who wants to get rid of their stock will sell at whatever price they can get, and maybe someone on the other side of the fence decides, "well, the last trade was at $38, maybe $37 is a good deal. Suddenly, the "price" becomes $37.

Now, there are still people who want to sell, but now, the guy that was willing to pay $37 is out of the picture, the next guy is only willing to pay $36. Since sellers are so desperate, someone will sell at $36. Boom, price is at $36.

Let's say this continues to the $31 range. There are far fewer people willing to sell their shares at $31, as opposed to $37, and there are more people who are willing to pay $31, who were not willing to pay $37. At the end of the day, there's a balance there. Anybody who's unwilling to sell for less than $31 holds onto their shares, and anybody who's unwilling to buy at higher than $31 does not buy any. The market decided price is now $31.

In reality, there are thousands if not millions of players, and the increments are a matter of cents. As the price drops, fewer sellers want to sell at such a low price, and more buyers want to buy at what they perceive to be a cheap price.