It's sort of a strange thing. An app like this should have the easiest horizontally scalable backend ever. Each user has no overlap whatsoever with other users (unlike more complex sharded instances like ebay, facebook, and every other social app).

So it must be more of a business decision about whether it's worth the cost of firing up all those new servers for a currently free app.

Makes me think they should have just charged for it out of the gate, then they'd have the financial incentive to actually fire up all those servers. Or a "subscribe now" to get to the front of the line.