When you say "I PM'd you", it's not clear, so, I think, Twitter, by choosing their own term, it uniquely specifies the communication channel, i.e. you can PM vi

When you say "I PM'd you", it's not clear, so, I think, Twitter, by choosing their own term, it uniquely specifies the communication channel, i.e. you can PM via different channels (like Facebook, etc.), but you can DM only via Twitter.