FB acquired them next year and if my memory is correct there were 19 in the team then.
With just a bit more effort you can see that most of those $148 are not related to the extra customer support we're discussing, but rather to the things that Signal is already doing
Costs and expenses in 2013:
Cost of revenue 53 (payment processing fees, infrastructure costs, SMS verification fees and employee compensation for part of operations team)
R&D 77 (engineering and technical teams who are responsible for the design, development, and testing of the features)
G&A 19
Besides, the original point was about huge$ from running a paid vs free app, which isn't the case