Distributed apps: where do you store large amounts of data?

I'm dreaming of the cool implications it could have to use a service knowing for sure which code is being executed.

I think I understand the basics of how distributed apps work algorithmically but I don't understand how you could sustainability deal with the amount of data.

Let's say I want to build a decentralized reddit on ethereum. Would all the posts and comments be stored forever on the blockchain? Or would you have to delegate some of the storage to an outside database and if so how do you secure the fact that it is decentralized and can't be brought down and does behave the way you think it behaves?