It really depends, am I friends with the person messaging me? If not, I would probably open the message to see what's up unless I can clearly tell that it's a spam/virus.
If I was friends with them, I would try to open the message and read it. But to be honest, sometimes I just don't feel like reading the messages and forget about them. (And when I say forget, I really do forget about them). Or I do read them but then never get around to replying because I get really lazy and eventually forget about it.
In all honesty, I know it's rude not to reply if I've read it but it's not like I do it on purpose. And there's no reason to block or ignore a person if they haven't done anything wrong ... Unless you find them annoying or something but then why would you have them as a facebook friend in the first place right?
This is off topic but, it's ironic that I don't even have a facebook anymore and it's been 3 years since I have. I'm speaking in terms of when I did have one though. And this post kind of relates to texting as well which I'm extremely bad at.
To be honest, a lot of people nowadays just add people to have more "friends" on facebook but don't actually talk to them or might not even know them in real life at all. It's really just a numbers game and it's quite pretentious. Now I'm not saying everyone is like this, but maybe the amount of "friends" they have on facebook is just too important to block someone. I mean, the amount of "friends" you have on facebook show people how popular you are right? (I'm being completely sarcastic here by the way). I'm sure you can see how distasteful I find social media.
I wish you the best of luck.