According to their website, 16,708,776 people have bought a copy of the game. At $26.95 per copy, that's a total of $450,301,513.20 in total income for however long the game has been available. I don't know if they sell other products or services, but $2.5 billion seems like too much for Minecraft UNLESS they are able to inject ads into game servers or create some sort of recurring income from those who have already bought the game.
Maybe giant public servers that rival WoW in terms of population? Maybe turning Minecraft into a social community? I dunno, but I'll probably pick up a copy and see where they take it.
EDIT: Left off a digit in the initial number of copies sold.