Most Helpful Guy
Not cool at all. It's dangerous business when they become friends with ur EX. However, it really depends how you and your ex fell apart or ended the relationship. Your SO and Ex could be up to nothing but, chances are they could be talking trash about you.
Most Helpful Girl
I would feel super weird about it, considering none of my exes are at all involved with either of our existing social networks and I don't imagine that really having any common ground...
Now, if I had dated someone who happened to still have a lot of my same friends and we often ended up at the same social functions, it'd be less weird if they became buddies.