I think there's two things being conflated here - checking in and bragging. It's quite possible to do either without the other.

I check in - usually foursquare rather than FB as it's useful for tracking - foursquare will tell you the last time you visited that location. My nearest and dearest like being able to tell where I am and given that I sometimes get confused by my schedule, they have no chance of merely remembering what I've told them.

I have friends, colleagues and associates all over the place. Posting my location on social media has triggered quite a few ad hoc meetings with folk that happened to be in the same location. I wouldn't have known otherwise.

I don't see it as showing off. Although in my case any comment such as 'here we are again' is usually more of a sigh than a brag.

But to each their own. I remember getting a kick out of landing my first F seat and taking a pic of my BP with a glass of champagne. I daresay I'd have posted it on social media if it had been around at the time.