OP is talking about "Facebook friends" specifically, which I think most of us can agree is a wildly different dynamic compared to the conventional "friend," or in your case, "good friends." You can be "good friends" with someone that's not on Facebook, so being a Facebook friend with someone isn't a necessary condition of being a "good" or even regular friend with said person.

I'm amicable with my exes, but that's because I don't really talk to them. I reserve nothing negative, and I would candidly tell someone who asked me about them that they're great women. But I don't call them to hang out with them, I don't call them to ask them for advice, I don't call them for anything, really; if I randomly ran into them, I'd say hello and have a brief chat. The frame of our relationships were emotional and physical, much more so than I have with other female "friends," and that attachment doesn't evaporate when I broke up with them. Myself included, people get jealous, people get angry, people have self-esteem issues. Remaining in contact, electronically or in person, doesn't serve a purpose because there's still too much emotional butthurt to deal with someone you loved but are no longer with.

Alot of other posts (not you Mitis, you never explicitly stated it) have suggested that OP's statement reflects immaturity - I couldn't disagree more. Defriending an ex on Facebook, in my assessment, is a testament to strong will and maturity. If you really love someone, you want to see them, and you want to remain in their life - even if just through some relatively meaningless electronic medium. To completely cut them out is something that takes alot of guts, and moreover it's a pre-emptive strike against brewing a fight or brewing more hurt feelings.

If I break up with a girl and de-friend her, she won't have the ability to see what girls are talking to me. Even if the cross wall-posts are innocuous, in her mind (I'm generalizing), she'll conclude I must be fucking them, and that I replaced her with them - not because she's crazy or immature, but because it's a natural reaction; when you're that in love with someone and suddenly it's over, emotionally you're temporarily scarred. If I deprive her of the ability to experience this, I'm effectively 'sparing' her that negative feeling, even if it means terminating my electronic association with her. So I sacrifice my friendship online with her (and probably in person as well), lose my ability to be able to be friends with her, to speak with her, etc., to spare her from those awful feelings. Applicable vice versa.