Factors To Consider
It's possible you're really goddamn annoying on Facebook — lots of stupid shared "joke pictures", confrontational political statements, mundane check-ins — in which case, who can blame anyone for unfriending you? You're a terrible friend. Friends should be interesting. Why weren't you more interesting? You brought this upon yourself, and now everyone is running away from you. Start being better and you'll start making friends. That's your best shot.
But let's say you're just a regular guy being cool on Facebook — posting the occasional fun photo, insightful link, amusing status update, thoughtful birthday wishes. You manage your Timeline like a pro! You're a good person, and a good Facebook Friend.
And someone still unfriends you. You have reason to be offended. When friendship means so, so precious little, taking the time to click enough to remove someone from your list of fake friends is hugely insulting. You're beneath fake friendship. Someone doesn't even want to see your name written down on an LCD screen. You're repulsive to them. Unfriending on Facebook is like being kicked out of fat camp for being too fat. And ugly. Ugly and fat.
But, don't forget:
Unfriending Is Healthy
It's a normal, perfectly rational thing people do on Facebook all the time. Most of your hundreds of Facebook friends aren't actually your friends, because it's emotionally impossible for a single person to have hundreds of friends simultaneously. Being unfriended might be someone simply waking up and recognising the absurdity Facebook thrusts us into. They're just attempting to rectify it, using features built into said social network.
We're all petty enough to care.
Let It Go, But Be Weird About It
If it's someone you actually know — a coworker, friend of a friend, some peripheral talking blob — you probably shouldn't make a fuss. Particularly, if they're connected to someone who is actually important in your life, it's best to just let the affront slide.
But some day, whenever the two of you meet in person again, you'll briefly lock eyes. I know what you did, and you know what you did. It's a moment of mutual panic, guilt, and animus. That'll quickly subside, and you'll go about your lives. Who knows, maybe you'll both end up forgetting the whole thing. You might even friend one another again. And so the cycle continues.
Or, if you feel like a fight...
Call Them The Hell Out
This part is fun. If you're not really worried what these people think about you, publicly expose what they truly are — cruel. Cruel jerks. They'll get their comeuppance. These unfrienders think you probably won't even notice, and if you ever do, you'll be too apathetic or nervous to say anything about it.
But bringing up the vile act — either online or in person — will shock them to their core. They'll be taken aback that you're enough of a weirdo to bring up a Facebook unfriending. Watch as they stammer excuses, stutter, and try to explain. They'll offer to refriend you. "It was a mistake! Oh, how did that happen?" They'll feel bad and weird and guilty. They'll feel something, which is better than Facebook's typical emotional feedback void. Sure, it's trolling, but it'll make you feel better without any kind of real moral transgression. The Internet should always be making you feel better.
This happened to me last year, when, after posting about a Facebook bug that actually revealed who'd defriended you, one girl I sort of used to know, who'd recently deleted me, reached out. Panic-stricken. Ashamed. But I forgave — as should you. Now we're sort of faux-friends again on The Book, and all is well. If anything, we're closer!
There's nothing wrong with a little revenge online. The consequences are so low, the results so ephemeral. What would've been a hissy fit IRL is as simple as sending an email. When all that's required is a slide and tap of the finger, petty loses the fight against pettier. You win. And then you can add the reconciliation to your Timeline as a Major Life Event.
User Manual is Gizmodo's guide to etiquette.