Ask HN: Should a startup own its data or be a layer on top of third-party data

I'm asking myself if a startup should own its data (text/pictures/videos) or use data from third-party like twitter/fb/flickr/etc and act as a layer on top of those services.

A lot of people are telling me that it's near impossible to grow a user base when you want your user to post a subset of the data they usually post on mainstream social network.

What do you think?