Just because someone signs up for a free service doesn't mean they're going to use and spread it. When you get them committed through paying, then there's an even higher usage rate as they figure out the best way to maximize it.

Under the original free plans we saw a lot of people just signing up for free to play around with it, only to end up paying a couple weeks later. By making it an actual free trial system, we're able to put a lot more pressure and messaging throughout the upgrade process.