Investors aren't buying stock in Facebook or pretty much any software startup (save yours) because they expect to someday get paid by the company. To make money off that kind of investment, they need liquidity. They all expect to see another financing round at a higher valuation, and with the size of Facebook these days, it'll probably have to be an IPO.

Regarding "Fashions change," I agree that that's the risky part, and from the amount of Farmville bullshit I see on my wall these days, it's a significant risk.