You can be "just friends" with him/her provided that both of you have truly moved on from the relationship and, most likely, are both secure in new relationships. Otherwise there will ALWAYS, and I mean always, be some sort of ulterior motive on the part of one of you to either recover the relationship or at least hook up and make things emotionally excruciating for both of you. Even when both of you know intellectually that you are not right for one another, there will be that id-driven compulsion. Even at the point when both of you have spent a significant amount of time apart, are in new relationships, and find yourselves able to communicate impartially again, why would you want to? Even then, you will never, and should not, get back to the point where you're best buds and are hanging out alone together all the time. The sad, sad truth is that once a relationship ends, there is no non-painful or truly legitimate way to preserve the sense of companionship and particular intimacy that characterized it and made it, at times, so wonderful. I learned that the hard way. Honestly, I think the best you can ask for is to be on good terms; to perhaps briefly catch up now and again, as it's natural to still be curious about what's going on in your ex's life; but as for an active close friendship, I think that (in most cases) is not an option following a relationship.