A big part of "ownership" for me is the ability to replace the service with my own machine, whether I actually do that or not. So (for example) I'd have less ownership over the gmail account <whomever@gmail.com> than one with a custom domain, even if I chose to forward everything to gmail or use google apps. I haven't looked into it at all, but my understanding is that it would be hard or impossible to point applications to a self-hosted dropbox replacement; compare that to, for example, github where I could replace it if I wanted to fairly easily (at least for the small, personal projects I keep there; this probably becomes less true for larger projects, so take it as an example of my thought process rather than a statement about the "ownership" of data on github).