This is the line at the Greenbelt Metro station on Saturday -- it curves off to the right out of frame. When we arrived and saw the full parking lot and the long line, we almost gave up on making it downtown...I suspect many people did.
Between the crowds, scheduled track maintenance, and a security alert farther down the line, Metro was totally overwhelmed -- it took an hour and half for us to get on a train after we got to the station. They started turning people away for lack of parking, and we only managed because I used to live across the street from where the Metro stop is now, and knew that we could park at a shopping mall a mile away. (While walking to the station from there, we met a woman who'd come all the way from San Francisco at the last minute, flying into Philadelphia and couch-surfing in Baltimore -- so comparatively speaking, I didn't feel that inconvenienced.)
Despite the overloaded Metro system, we made it down, but not until after all the performances were done. Still, we were on the Mall there before the 3pm "official" end time, so I'm counting it as attendance in my book. Many people snapped photos of my "Radicals for Moderate Discourse" sign.
The crowd was amazing. Attendance is estimated at 215,000 based on aerial photos. (The same source estimates that Beck's recent gathering drew only 87,000.) But if that's only counting the maximum number of people on the Mall, it's a gross understatement; many us of arrived late due to the Metro being overwhelmed, and many people arrived, saw the crowd, and realized they weren't going to be able to get anywhere neat the stage and either just hung out in the streets, or stayed for a bit and headed out early. So there was a lot of flow in and out. And had Metro been able to handle the influx, I think attendance would have been even higher.