Is that it? Is that the ultimate metric by which a developer or entrepreneur measures success? So what if you're contributions are going to be used by a billion people - Facebook is fluff, an advertising supported business where the product is free to use and the usage data is mined for revenue.
I feel like what they're really saying is "Facebook wrote me a giant check that I couldn't refuse" - why is that so hard to say? I don't think many working for Facebook leap out of bed thinking they're going to change the world - but your startup might.
Man up, admit when you're being paid enough to silence your discontent or burn your old bridges for the city on the hill.