Actually, there's another way to hit >= $100M.
Be a strategic acquisition target. (Route two: The viral effect can overlap with this.)
I forget who blogged this (Dixon?), but the idea is that that to hit a >= $100M valuation, you should not be thinking of the standalone value of your business, but instead the value it provides to potential acquirers.
Sitting on crucial patents is one approach, but extraordinarily tricky.
More importantly, if your existence presents a strategic challenge to a larger player (e.g. Instagram), then your business is worth a lot of money.
-- They have the potenital to destroy N-times more value than they can create.