Not sure how you're figuring that. At best - 1.5Bn users locked in - you're at less than $1bn _revenue_ (after you net-out the 30% that goes to Apple or Google for an in-app transaction) and much lower net profits (presumably, it costs some fraction of a dollar in ops, bandwidth and cpu to support a paying user for a year.)

Also, even at $1bn in revenue, this is 16X sales... that's usually the kind of price you'd see for a company that can still grow a lot. When you have 1/12 of the world's population as users, and you're capped at $1 per year per user, there's not a lot of places to grow. Clearly FB valued this based on strategic value to their broader platform.