If you leave a comment on my LJ then I can get to it from work, and write a decent reply using a proper keyboard, when I have the time.
If you leave a comment on my DW or Facebook then I can only get to it from my phone, and will probably write a terse reply using Swype, which is awesome, but still slower than a full-size keyboard.
If you reply to me on Twitter and a well thought-out response would take more than 140 characters, then I'm not going to reply at all because multiple-tweet statements are the spawn of the devil. If it requires a link, or anything else complex, I might reply when I get home, if I can be bothered. If a simple response will do then I'll respond via my phone, but frankly I find Twitter conversations to be a right pain.
Original post on Dreamwidth - there are comments there.