Expiration dates for posts is a brilliant idea. Who ever is working on the "next Facebook", I hope you implement something like this.
In my mind, ideally the next big social media platform will be P2P and open source. I have some ideas on how this might work and the features this would include, but I've never hashed them out since my programming ability doesn't approach the level required for such an undertaking. Maybe I will as a thought experiment and publish my ideas so that anyone can use them.
And there are numerous nodes you can create an account on -- no need to be self-hosting. Though with FreedomBox, that would actually be pretty tenable.
One of my friends from G+ (which is to say: someone I met through G+ and started following there) has made a practice of bulk-purging his account every so often.
My own response has been to greatly curtail my activities on the site (due to Google's ever-encroaching property linkages), even though I'm already participating pseudonymously. I've hand-deleted some content, but am holding out on a broader deletion given the prospect of recovering some of my comments (presently not included in the Data Liberation Front), with some third-party tools. Though I may just pull the trigger and do a full wipe.
The fact that Google offers this option is actually somewhat commendable.