price to earnings for AAPL is around 30, which means that if you bought all of apple, and it continued to make just as much money, it would take 30 years to earn your investment back.
Equity is different from debit, yes, but when you talk about how much money you are making, it really only makes sense to talk about how much money you are making based on how much money was invested. If you have 1mil invested, and you are making 1 mil a year, that's pretty goddamn impressive. If you have 750 million invested, and you are making 1 million a year profit, well, gee, I could do better than that buying bonds.
Now, Both AAPL and facebook have the P/E ratios they have because people expect them to make more money in the future than they are making now. For instance, if I was to sell my company (and I've had a few offers) it looks like I'd get about a year worth of revenue. This is pretty typical when buying ISPs, because nobody expects exponential growth to go on for any period of time.
Just sayin; It is a reasonable and well-established idea to look at how much equity is invested when looking at a company's profit margins. It's also reasonable and well-established to look forward at how much profit the company is expected to make in the future, too, which is why facebook is valued as it is by some people.
(there is, however, a great degree of uncertainty in projecting what profits a company may make in the future, which is why there is a great deal more controversy over the value of twitter than over the value of AAPL. Personally, if there was a convenient way to buy a long-term 'put' on twitter, I'd do it. Just my opinion,)