According to Wikipedia, "Ridership as of the fourth quarter of 2009 is just under the projected 5,830 to 9,037 persons per day traveling on the line that was predicted."
It's not that the ridership is bad--it's just that it could be so much better. The system is really designed to handle crushing loads. FrontRunner uses bi-level Bombardier cars. Each car can seat about 360 people. Assuming each train-set consists of 3 cars, that's about a thousand person capacity per day. Trains run hourly, with some additional peak service, for a rough total of 60 trains per day. Thus, without adding additional cars, FrontRunner could theoretically handle 60,000 riders a day. That's about twice as many as Bangerter Highway currently carries.
But FrontRunner was built on the basis of projections made as the housing bubble was inflating--it looked reasonable to assume that all of Davis and Weber Counties were going to be converted into housing developments within 10 ten years. When the housing bubble explode, that plan went to bits.