Steam, which has 65 million registered users, just had 7 million people online at the same time. That's a lot of people at once. That's like if everyone in Hong Kong, or most of the people in New York City, all got onto Steam at once to play games. That would be one crazy LAN party.
Last year, Steam had a high of 6 million concurrent users, so it has grown by 1 million in a year, or about 16%. That's a lot of growth in a year. Almost a half-million of those people are all playing DOTA 2.