That might have been the spark that lit the fire but I believe that people who are NOT celebrities that tweet actively, do it to reach a broader audience and engage in conversation with others in their vertical. This is in stark contrast to the network you would have in Facebook which is much more intimate.
Twitter is the internets water-cooler conversation.
Facebook is the conversation you have at home with your family and friends.
Twitter is the internets water-cooler conversation. Facebook is the conversation you have at home with your family and friends.