This argument flares up over the years. We used to have a poster who put the ratings up regularly, it was pretty shocking how weak TFC regular season ratings were, mostly 50-100K iirc, no better than darts. They would flare up to 500K if they had a strong position in the schedule (ie a Saturday 5pm game right before or after an attractive HNIC playoff game)
Everyone agrees the potential is there, but I think the reality has been pretty disappointing. Unless it's a semifinal or a final, TFC mostly draws flies on TV. So far.