Comment by PhilWright

The problem with your personal data store is that it will be impossible to stop the large companies like Google/Facebook from simply copying and caching a copy of all the data they need to access. You give them permission to handle your email and calendar information and bingo, they have a copy of your entire set of emails and appointments. If you use multiple services from Google/Facebook then they will end up with a good copy of your entire dataset and can mine it of value just like they already do.