It is quite surprising that some people really haven't caught on yet. I look back at some of the things I posted on superficially private forums in the late 90's that in retrospect, I'd never post today. I don't excuse my naivete but still, I think a lot of people (both young and old) hadn't quite wrapped their heads around the capacity for indexing and preservation of
all that data. Sure, you didn't want your stuff showing up in a search engine but hey, you used a screen name!
Regardless, however misguided my ignorance may have been in 1998, I seriously don't understand how anyone would be OK posting things that could easily be incriminating (photos of their stash, videos of drunken behavior) on sites as public as Facebook.
As you mention with your quote, maybe it's a reaction to the idea that you'll never be able to curate the sober and responsible "persona" you'd like without unrealistic levels of precaution. So rather than spend your youth in constant fear and anxiety that someone will see your drunk pics, there's something liberating about just admitting that we're all human and many of us have our "responsible life" and our moments of cutting loose.
Still boggles my mind when people post blatantly illegal activity on Twitter or Facebook though. But other than that, I try to keep a balance. When I'm out drinking with friends, any photos or videos that get taken do not go on Facebook and if they are shared at all, it's only with the people who were involved and only via less public channels. And if the behavior in question is more potentially objectionable than some drunk karaoke, the cameras stay away.