$1 trillion?
This makes no sense at all. There are no published accounts I can find for Facebook, but Wikipedia (may or may not be correct) gives it a $2bn income. Google has (same source) an $8bn PROFIT.
Company valuation is difficult, that's the problem. As a user above notes P/E ratios are a good measure, we don't know Facebook's E though. Just assuming (wrongly) it converts all that income into earnings, $1Trillion would be a P/E of 500. If the user above stating a $300M profit is right, that jumps to an eye-watering 3,333 (as another notes). That's a bit meaningless, so let's look at Google's, which is about 14 (future) or 20 (trailing) per Yahoo Finance, which is very close to the mean of a traded company.
At $1T value, and a half-sensible P/E ratio (say 20) it's "profit" of $50Bn PER YEAR. That's $8 for every man woman and child on the planet. Most of whom don't have that much to spend in total. It's 1.5 times ExxonMobil - a company with lots of assets and pretty much a product the world cannot function without.
Still looking sensible at $1 trillion? Even at $75bn it's a P/E ratio of over 200. Feels wrong to me.
When you look at Facebook's revenue - it's banner ads and games (I don't know how the latter works). It's click through rates are very poor compared to Google. That's key - I go on Google and search for "laptop" that's because I want a laptop. If I see "laptop" alongside my Facebook page (harvested from some data of mine) I may well ignore it.
Facebook's user base may well be about as big as it gets, I think it has started to fall, even though Zuch has said he doesn't care (recent Reg article). Its demographic is, I understand, kids. Kids don't have much cash, even though they may click on ads. If you go subscription, they go too.
The article says the success and value is due to it being open source, I think, I may have missed the point somewhere I was laughing so much at the valuation, and that had encouraged developers, which drives the value. Developers seem to help out at that famous open source outfit Apple (see the App store) and another one, say Microsoft.
In short - it's not worth $1 trillion. It's not valuable because of open source.
The value is jumped up by some self interested traders who have sunk billions into it, and want to get the cash back when (if) it IPOs. They must hope The Next Big Thing does not appear before that happens.