I suppose this is because there are two outcomes for these huge user bases like instagram and twitter. 1) Get bought by a larger company that can use add your

I suppose this is because there are two outcomes for these huge user bases like instagram and twitter. 1) Get bought by a larger company that can use add your user base to their total user base 2) Start advertising intrusively and hope that you can make enough before your service is no longer favorable Instagram comes to mind for #1. I'm thinking Twitter will go with #2 when investors get sick of no returns year after year.