I remember when I was high school, all the kids used to find gf's from the same parties, so in each party there were fights over ex's. It was pretty usual to dump one person to go out with his/her friend.. It's a bit awkward, but I've seen it happen later in life with some of my friends who have tight circles. They'd be at a party hanging out with one person, then swap 'em for the next.. And everyone pretends it's completely natural.
Anyways, I'm in good terms with my ex's. I've actually briefly met some of their current BF's, too, without too hostile feelings - not that I've hung out with them or anything. I actually am having some coffee with my latest ex, in a couple of days. No, just coffee.