IIRC There are mass-telecommunication centres in some western countries that process a huge %age of the data traffic sent by email/web/phone/etc searching for specific words or phrases to do with various illegal things. I'm sure I read that on here or on anotehr forum site. It was backed up with some proof too, but I can't remember what.
Emailing something like "Bob, can you bring round that 1/8 you've got in your house please" isn't going to be worth the Law's time. Constant back and for emails like "I've got the half ton of TNT. Jim's packed the van full of it and we'll drive it into the Government office in town at 3pm" might be picked up by the system if its part of a long term correspondance.
I've discussed buying drugs via email or text messaging. I try to do it as little as possible obviously but once in a while wouldn't be noticed probably.
Small time people aren't worth a huge investigation shifting through terabytes of information looking for a few things to get enough for a warrant or arrest. Large time dealers / terrorists etc would have more sense than use such open means of communication without at least having some form of code.
Maybe there's a DEA agent reading this very sentence right now. If so, Hi

, its a nice website isn't it. I'm sure if they were really bothered they could trall through the entire site, including archives looking for every post of mine, or yours or anyones (who have a high enough post count) and narrow down who we are to less than 10 people, maybe even to us exactly, but it would take too much time and resources to do that and then to come and arrest us. Besides, where's the proof I've taken drugs? I could just be lying to make make myself feel big on a website in front of strangers. I'm sure there are some people on here who are like that.
For all you guys know I'm a DEA agent myself who just reads other peoples posts and makes up posts about myself to fit in by modifying stories I've read. I might never have taken E or Speed and just lie about it so you lot feel comfortable talking to me.
I'm not though (no really, I'm not!).
Maybe they are watching me now.......
*looks through blinds, cleaning sniper rifle*
Doesn't matter, I'll put on my tin foil hat so they can't read my thoughts and when they come for me they'll never take me alive!!!
*cue evil laughter*