With an estimated 700,000 boardings per day, 1 out of 10 Muni riders jump aboard without paying. (For shame! Two dollars is a bargain for this kind of entertainment.) According to a report by SF Chron, "[f]are evasion on Muni occurs most frequently in the afternoon and at night ... Among the lines where the problem is most prevalent are the 9-San Bruno, 14-Mission, 38-Geary and 47-Van Ness."
But, as the report goes on to point out, riding without paying happens on all Muni lines, frequently. Fare evasion, it seems, "is enshrined in Muni culture," a cultural affect that will land you a $75 fine.