Yep. When my ex discarded me my very dear and good friend who introduced us also got discarded. We were both immediately un-friended on Facebook when she reactivated her account. Ironically, she sat next to him in a movie theater at a MeetUp Event after all of this went down. These people are inconsistent and have poor boundaries.
Same happened to me some time after the breakup: she made friendship with 10+ of my friends during the relationship (it lasted 1,5 years), and added them in FB over time; let me say that my friends really liked her - obviously, they didn't know about her temperamental issues, with the exception of 2 friends of mine - and it seemed that she really liked them. So, no reason to unfriend them.
However, Some time after the breakup, she proceeded to unfriend ALL of them in one shot, like as if she had to close a chapter of her life.
Let me say that we had a medium-distance relationship (lived 250km apart), so overall this didn't affect her - my friends all live in my city.
However, let me.say that it is SO mind boggling how these people can detach so quickly and completely, even of we're talking about long-term relationship... .it's really disturbing.