It's part of the tradeoff of being investment-funded. Investors offer you money based on an estimate of the future value of your company. When you take the money, you now have to get the company to at least that value for them to be able to make money on their investment by selling their stake. If you can't make that happen, the investors will be very unhappy with you, possibly up to the point of using their ownership stake (or joining with other dissatisfied stakeholders) to have you tossed out and replaced with a "better" manager in order to "save the company."
In other words, "large and profitable" is enough if you're organically funded, but if you're investment-backed you have to not just be profitable, but at least as profitable as your investors predicted you would be.
In Facebook's case, those predictions could be quite large. On the pre-IPO end of things, Goldman Sachs' 2011 investment in FB, for instance, was premised on a $50 billion valuation (http://dealbook.nytimes.com/2011/01/02/goldman-invests-in-fa...) of the company. Depending on whose numbers you believe for the total number of FB shares outstanding (see http://www.businessinsider.com/facebook-shares-outstanding-2...), at $20/share their current total value is somewhere between ~$40 billion and ~$55 billion. The high end of that estimate is north of Goldman's valuation, but the stock won't have to fall much farther for Goldman to get nervous.
Similarly, when the company goes public, now you have "the markets" to deal with as investors as well. Just like VCs, public investors buy into a stock at a certain level because they believe that level represents a value less than the company will eventually be worth. And also like VCs, if your management makes that bet fail, they can and will organize to remove you, or at least make your life difficult with an activist board or other oversight mechanism. When Facebook opened for public trading, the market estimated its value at around $104 billion (http://dealbook.nytimes.com/2012/05/17/facebook-raises-16-bi...) If you bought in at that price, your investment today would only be worth a little more than half of what you paid for it. That doesn't make for happy investors.
Investors want to see a return on their investment, and they prefer to see it today rather than tomorrow. Corporate managers who want to stay corporate managers do everything they can to deliver it, and sooner rather than later. Hence the focus on short-term metrics like quarterly profits over long-term sustainability.