Bitcoin has value at $500 because someone thinks its value is such that. Obviously it has no real value. But either does a stock. Technically a stock is generally worth ~1/100th of a cent.
Not at all. As I explained above, if you own some of a company's stock, you, quite literally, own a certain slice of the company - its factories, machines, patents, organization, brand, - and of its future profits. The price of one share (which depends a lot on the company) is a function of the value of the company's assets, which are lised and evaluated every quarter by independent auditors; and of its expected profits, which can be estimated from concrete market and production data.
How about gold? Why is gold over $1000 an oz? It doesn't keep you warm, you can't eat it, you really can't build with it. It's worth that because its used as a hedge and people perceive it has value. Not very dissimilar to Bitcoins.
Unlike bitcoin, gold does have some utilitarian demand besides currency, investment, or speculation (jewelry, electronics, etc.). It It also material (everyone can see it and many can test it, without the need of sotware), is permanent (won't disappear if some network stops working), is unalterable (its nature cannot be changed by some community changing some protocol), and is understood by most everyone.
But, indeed, gold is currently is overpriced, because its demand is mostly from investment/speculation, and is much higher than the utilitarian one. Like bitcoin, its price is largely pegged to itself, and could fall drastically if the mood of investors changes.
The changes are slower for gold, because there is a much larger market, and because the trust people have in gold's future demand is not as easily changed by news as their trust in bitcoin's future demand (which can change radically by a decree from a government, for example. Nevertheless, gold has lost about 1/3 of its price between 2012 and 2014 (from 1800$/oz to 1200$/oz).
By the way, this drops is why there are presently so many predictions of imminent global economic catastrophe: they are usually precious metals brokers trying to convince people to invest in gold and silver again.