Per hour rate seems somewhat misleading given the split shift. There's 15 hours between the start of their work and the end of their work, and the gap in their day is short enough that they often can't make great use of their break. If you were to consider all of that hard-to-use time as time on the job (since the job is the reason it's being underutilized), it would cut their effective rate almost in half.

Imagine someone hired you for a job where they wanted you 10 minutes out of every 20--10 minutes working, 10 minutes off, repeat. You might have a day where you clock 6 hours, but it would essentially be a 12-hour day at a job with a lot of stops and starts. Whatever rate they claimed to pay you would be inflated by two because they were only paying you for half of your time.

I'm guessing something that granular is illegal, but the split shift acts similarly; there's a big difference between working a 9-hour day straight (say with one 45-min meal break) than working a 9-hour day with a 6-hour gap in the middle, so the latter should be more highly compensated.

And so it's not necessarily an economic conundrum. There's no magical market law that says there is no bus-driving labor that could be worth more than $20 per active hour driving. Bus drivers want better compensation (or at least accommodations that make the gaps more useful to them, such as bunk beds). Facebook is surely willing to pay some amount more than they are now to continue the bus service (evidence: increased costs such as SF's new pickup fee have not stopped bus service from growing). So Facebook could pay more due to increased driver compensation or accommodation cost, the drivers could in turn be paid more or be better accommodated, and everyone would be getting more out of the economic deal than they're putting in.

The union would do what unions always do--shift some of the economic surplus that the company is currently enjoying (paying less for the bus service than the value it provides) to the workers.