I also returned an hour ago to post my first of the week! :)
My way of dealing with the permanence of information on the blockchain is to carefully write everything so as not to reveal who I am. That way I can just switch to another account and people will always know something shameful about... a fictitious character? I think transparent blockchains somewhat incentivise anonymity.
As for social networks, I barely use them. I only use Steem because it's my "home" and it's very pleasant, and it's the only social network that feels truly "mine" (I own a part of it with my tokens/shares and no one can censor me!). I don't have a Facebook, Instagram, and only recently created a Twitter account to promote my Steem content, but I'm bored of it already. I can't enjoy social networks in general.
I feel like PoB is, as you say, easily cheatable. and it's sad, but at the same time I don't have any idea about how cheaters can be defeated apart from a mixture of SteemCleaners (flagging) and curation like OCD (upvoting) to discourage abuse and encourage positive participation. It can be thought over and planned out, and I think we're slowly but surely making progress toward a nicer place that can be truly called the future of social networks.
It's gonna become interesting! I wonder what the rate of "returning accounts" will be compared to "new users" weekly once this place continuous entering main stream usage.
Returning as in people who left a long time ago but come back once they see their "failed social network" becomes big enough to be relevant in their lives again? I don't imagine that it'll be a very big percentage, but who knows.
We could build a bot to check it haha, every time there's a post, read if the user's last post was more than 3 months ago or if they joined more than 3 months ago. If so, add the user to returning user. If the account is less than 3 months old, new account? I don't know. Metric tags are subjective!