Before the advent of Twitter, most educators I know had limited opportunities to collaborate with colleagues outside their building. Some subscribed to listservs or participated in online forums, but these outlets lacked critical mass; teachers also networked at in-person conferences and training sessions, but these isolated events didn't provide ongoing support.
Enter Twitter. I've heard many educators say that Twitter is the most effective way to collaborate and that they've learned more with Twitter than they have from years of formal professional development.
Via Steven Engravalle, Paula Naugle, KB...Konnected, Jorge Reyes