I don't think many people know it actually exists. Bus services in general are a confusing mess in Melbourne and poorly promoted. I wouldn't be surprised if over half of Melbournians didn't know about this option. Interstate and foreign visitors have no hope.
Of course, MEL isn't being very helpful here. Not sure how they can complain that the bus service doesn't get enough passengers when they've relocated the only bus stop out to East Adelaide. The bus stop needs to be moved BACK to the main terminal complex immediately. PTV can then help out by doing some decent promotion of the Broadmeadows+901 airport link. Of course, SkyBus will throw a tanty, but PTV are meant to be serving public transport passengers, not a private bus company. SkyBus could come to the party on this (if they really want to) by falling into line with the standard fare structure under a deal with PTV.