@Takes123,
Nostalgia.
Seriously.
Facebook and other social networking sites do this. They feed our nostalgic impulses by delivering images of old friends. And our friends curate their content. After all, most people don't post images of themselves with stained shirts, or a hangover, or in the midst of being angry. You are seeing a rose-colored view of their life. And, in return, you're offering one up that's just as rosy.
If you really want to kick this feeling, agree to meet your friend. With your husband. I mean, mention it and everything. "Oh, and Craig (or whatever his name is) will be joining us. He's excited to be meeting you."
See what happens. If the ex is gracious and has no ulterior motives, he'll gladly agree to it, and might even suggest bringing his SO along as a fourth. Or your ex might balk, suddenly backing out of things. You will then know that his motives were/are less than pure.
I don't mind telling spouses this sort of thing, because it's honest and open. This is not for you to be comparative or do any bashing. Instead, it's "Craig, I've been in contact with Don, my ex, on Facebook. I'm sorry I didn't mention it earlier. I thought it was harmless, but I am mentioning it now and giving you full disclosure because I want there to be no secrets between us. He has actually suggested meeting and I mentioned I wanted to bring you along. And I do! But if you wouldn't want me to go at all, with or without you, I understand your feelings and will respect your wishes in this area."