So, the relationship comes to an abrupt end due to nothing on your side. You are confused and completely thrown off. Then you say the magical line, "We can still be friends." Out of the hope of still communicating, you said "of course," but new flash; you don't have to.
I am so sick of being expected to still remain "friends" with your ex of any sort. Sometimes, it's completely appropriate to still be friends, but the majority of the time you are going to be more hurt than if you just walked away. You have every right to decline the right to be friends with someone who doesn't deserve to be your friend!
You don't need to continue to follow them on every (or any) form of social media so you can work yourself up over their latest post. You don't need that in your life. You may be convinced that it won't bother you, but the second he calls a new girl "this girl" in a caption, you're going to be broken.
You're going to wonder what she has that you don't. You are going to question is she prettier than you. You are going to ask your friends if they know anything about her, so you can feel better about yourself when they say she isn't a good girl.
When he text you and ask how you are, because you're friends and that's what friends do, you're going to say good while you're tearing up; while you wish you were as happy as you were with him.
So next time they say, "we can still be friends", politely decline the already fake offer. It will benefit both of you in the long run and maybe one day, you can truly become friends once feelings have stepped out of the equation, but until that day there is no shame in not wanting to be friends.