My understanding is that a big source of ADP's profits is their ability to make money on the float. [0] Does ZenPayroll do this too? I suspect it doesn't make sense at the current scale, but could be part of their valuation. Payroll is a great recurring revenue stream. Even with these new entrants, most firms keep the same provider for many years. (I don't pay great attention, but I don't recall ever having a paycheck come from someplace new within a given company)
[0] http://ww2.cfo.com/people/2013/04/how-adp-turns-payroll-into...