I imagine we'll see an incentive program soon..."Refer 5 friends and this month's bill is free." If you refer the 5 friends you talk to most, the company will save much more than $19.
VoIP is cool, but traditional phone calls and text messages are not always ideal. I think innovation on the communication channels themselves would go a long way for this company. Think http://voxer.com
Facebook is in a particularly good position to pull this off correctly. They already have a "cool" factor, and if they could come up with a communication tool that people really like, so much that you're jealous your friend has it but you don't, the people will come. Blackberry pulled this off for years with BBM, but lost its "cool" somewhat recently.