While it’s generally a nice idea to stay n good terms with your exes, it’s debatable whether staying Facebook friends with them is ever sensible. Firstly, it means you can each see new relationships and start making unhelpful comparisons about your lives and how fast you’re moving on.
It also means that, if one of you isn’t completely over the other there’s the opportunity for over-doing the “staying in touch” thing. 4 months of no other contact is a long time to keep tagging someone in “hilarious” memes. I think that Dan needs to chill.