I'm going against the grain here slightly. I definitely don't go through all my posts to see who liked them, so I can go and block anybody who's unfriended me, and I think that's a bit over the top.
But I've only got about 95 friends at any one time, and I know the exact number as I go on Facebook most days. (As well as a few ex/current neighbours, ex/current work colleagues, and old and new friends/acquaintances, I've got got family and friends who live far away and I keep up with contact with them on there.) So if I see it's dropped from 95 to 94 it doesn't take long for me to figure out who's unfriended me, as I pretty much know who is on there. And yes I will block them.
The reason being is because they've made it very clear they don't want to be friends with me - so why would they want to see my Facebook again? And also I've had people unfriend me before on Facebook, and unfollow me on Twitter (so I end up not following them them by default.) And then a month later they decide to follow me again on Twitter, or try and 'friend' me again on Facebook, and I think WTF? 
I really can't be bothered with nonsense like blocking/unfriending me, and then trying to friend me again a month later when they're in a better mood or when they can be bothered with me again.
So I will block them yes - for different reasons - because I've been unfriended and blocked in the past and then people have tried to follow me, and 'friend' me again. If they blocked/they unfriend me that's it they're gone. Yes I'm fully aware I sound like a petty eejit. But you know what? I really don't care.
~@PJames84 YANBU. You do what's best for you. 
The great people of Mumsnet will not say the same though. Blocking people -for no other reason than they have unfriended you, (according to most Mumsnetters) is pathetic and petty. In real life, many people do it. This person has decided you're nobody in their life now. All you're doing is returning the favour. Ignore the 'oh I am not so petty and pathetic, and I can't imagine having such a sad empty life' type comments, and people claiming 'even my 12 year old thinks this is embarrassingly immature!' type comments OP. We've all got a bit of a childish and petty 'fuck you!' streak in us. Yes ALL of us! 😄
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