A related point made by Zeynep Tufecki is that mass protests in the past used to be show of force. If you could get half million people to travel to Washington and participate in a mass protest, that meant these people cared enough to take time off from work for a few days, spend money on travel and participate in a policitically and socially risky activity. It also meant you had several tens of thousands of organizers in diverse parts of country who managed the logistics of getting so many people to come together for the protest. The implication was clear -- give us what we want, if you don't, we definitely have the votes to take you down.
In the age of Twitter and Facebook, protests are a lot more convenient and a lot less difficult to organize. It is not clear that half a million people protesting in NYC or DC means much in terms of actual electoral consequences. Whether they can articulate it or not, politicians definitely know this and react accordingly.