For the initial stage, your schema will probably be in a flux and ONLY for that reason, I would suggest using MongoHQ or MongoLab on Heroku. After a while, if you decide otherwise, you can always go back to Postgresql.
OMG! The 13 guys at Instagram are not going to end up with approx $100,000,000 each. Instead they might get something around $22,000,000 in cash and more than $50,000,000 in Facebook stocks. I hope they can survive on that meager earnings.