This paper tests the hypothesis that social information provided by the internet makes it possible in large groups to exert social pressure that Olson considered viable only for smaller groups. In two experiments - laboratory and field - subjects could choose to sign petitions and donate money to support causes. Participants were randomised into treatment groups that received varying information about how many other people had participated and control groups receiving no social information. Results suggest that social information has a varying effect according to the numbers provided, strongest when there are more than a million other participants, lending support to the social information hypothesis and to claims about critical mass and tipping points in political participation.