How can I automatically delete the oldest Photos which are uploaded to the Cloud (Google+, Dropbox, …)? When I was shooting photos mostly on my offline point and shoot camera, there was one obvious way to manage the pictures: copy them to desktop via USB, then delete from camera. This was typically automated using picasa out similar.

Then came the smartphones. You can still use the same technique, but Google+ and dropbox offer instant upload for backup purposes. So everything I shoot is copied to the cloud, but I still keep it in my phone. I can delete, but that's not automated at all. And I have to be sure not to delete stuff that wasn't yet uploaded. 

Am I missing something here? Is there an obvious solution to this problem? Or is everyone fine with using USB like with regular cameras?
Dropbox's new Carousel app has an option to "Free up phone space".  This will delete photos from your phone that have been uploaded to Dropbox.

It's not automatic, and perhaps not exactly what you want, but if the real problem is running out of space on your phone it will probably help.