@elessar
What does irritate me is the type of people (and I know a few like this) who will message saying something like 'hey, want to do dinner soon? Are you free in the next few weeks?' And I'll reply saying 'yes definitely, free x, y and z or let me know when you can do and I'll see' and then just don't get a reply. It's pretty infuriating!
Agree with this. Pisses me off. Especially when you text them back within 2 minutes, and 4-5 hours later, they still haven't texted back!
It's like 'you texted me 2 minutes before I texted you back. You must have still be near your phone!' 
So you end up texting them again and again, and then end up ringing them, (and they don't answer of course!!!)
Then when they do answer your text that says 'ok I can meet you on the 18th, 19th, 20th, or 21st; can you make any of those dates,' they text back with 'sure, the 18th or 19th is good.' So you are STILL not sure when you're going to meet them! 
So you text back '19th then!'
NOTHING. For hours.
Another thing that rankles me is when people don't text back, and yet they keep putting posts on twitter and facebook. So they're on their phone, but can't answer my text!