There are multiple aspects. There's post-depression blues, https://en.wikipedia.org/wiki/Post-vacation_blues , there's the excitement that comes from new experiences, funded usually from savings and not having to work, there's the difficult of talking about the experience without sounding like bragging. There's also the problem that at home you need to work more towards long term goals, which don't have the same immediate gratification.

As for FB, don't look at me - I stopped using it about 8 months ago. But my belief is that travel accounts bore people. That's why the travel shows (in the US) are often on public TV stations, and not prime time commercial stations.