It could be that they haven't fully moved on and are keeping tabs on you. Or perhaps it's just a habit they haven't broken. Maybe they still care about you on some level and this is their way of staying connected, even if it's in a passive way.
Well, 'ex' is probably your former partner. They might be watching your stories out of curiosity. Maybe they still care about what's going on in your life, or they could be just nosy.