@Michelyne,
We get a happy high from nostalgia. It's a cheap high, actually - we all look back with rose-colored glasses, often because we were younger (of course), prettier, thinner (often), had fewer responsibilities (usually), etc. This is why you see it everywhere. It also has gotten over the 'get to know' hurdle. You already know this guy, so the initial hurdle of knowing him is already overcome. Getting to know the guy down the street is harder because you have that initial hurdle to overcome.
Envy is another powerful drug because we get a high (but it's not a pleasant one; it's an itchy, twitchy stressful one) from comparing ourselves to everyone and everything.
So - together - they are powerful, and FB in particular exploits that.
Just tell yourself, as people used to say -
he's not all that.
Thank God FB etc. doesn't have smell-o-vision. You don't have to deal with his dirty socks.