I agree with that theory about online gushy couples breaking up. I know at least 3 now divorced couples who used to gush all the time on FB about how great the other spouse was and post photos of their spouse and saying how gorgeous they were. Couple of months later - divorce.
Sometimes I am a cynic.
In such a mode I suspect that behavior is a try to "sell" their partner to someone else so that they have a "he was cheating" reason.
In my experience it more has to do with those couples being more volatile and often more immature in general. Oooh I love him he is the one for me! to "I can't believe he left the toliet seat/forgot our anniversary!" to "I love him so much" etc. I find those people exhausting.
My husband TOTALLY forgot our anniversary this year. To be fair, I remembered the morning of, so I just made a relatively nice dinner and figured we'd call it even, but then he walked in and was like 'wow, this is nice, what's the occasion', and like... the door was open, I just COULDN'T, so I said 'well, DEAR, it IS our anniversary' and I have never seen a face that was the picture-perfect definition of 'oh shit' before. Hilarious. (There was a lot of laughing at that face, because maaaan...)
Neither of us are big on date-related romantic gestures, though - I mean, I expect (and recieve, and give) love and affection on a daily basis, and also general recognition and appreciation, and joint work and problem-solving and all the stuff that makes this work. My expectations are way higher than 'remembers the anniversary' or 'bought flowers', and they're also less sharable-on-social-media, status-symbol-oriented.
(Swear to god, the most romantic thing he's ever said to me was at 2am, the night all the kids got stomach flu and spewed in arcs over the room, so we'd showered them and tucked them into our bed so we could deal with the biohazard that was their room, and he looked at me, exhausted and grossed out and almost keeling over, and was like 'well, there's no one else I'd want here with me. Thanks for being here'. And that, for me, is a good relationship - if you can find a way to demonstrate affection and appreciation through THAT, we can build something really solid, whereas 'remembering an anniversary' is a google calendar reminder that means eff all to me in the long run.)