There is no such thing as "being friends" with an erstwhile romantic involvement. That is all bullshit. There are friends, and there are lovers. One can end up in love with their friend, but how the hell do you demote someone from lover to "just friends"? Yep, you end up staying in touch, partly because you care, and mostly because you want to know what is going on in the ex's life. Like how he is doing, where he is going, and also, is he seeing someone else yet? Is she fat? (Oh, actually, that's a given). Are they serious? Is he getting married??? And when that is discovered, you don't really care any more. It doesn't matter that you have gone through two and a half more romantic involvements in the meantime, or even are yourself settled with three kids and a pati parmeshwar. The curiosity remains, and once satiated, it no longer matters. From then on, it's a predictable future- either the newly-weds drive off into the romantic sunset, make babies and all of that- or things sour (because she is fat, perhaps) and they part ways. Then keeping tabs on him becomes the fat girl's headache, not yours.
So it always bothers me when I hear of amicable splits and that "we're still friends" bullshit. We need a new word for this kind of civility- I say we call it "Ex-ship".