People did pay. Its just that the charging was somewhat selective. Obviously you and your friends weren't in the group that was selected.
Originally, it was a paid app ($1) on the iPhone. Eventually, that was dropped in favor of using in-app billing to actually charge the $1/year on Android... but this was only enforced in a few selected countries at first. Once the Facebook acquisition happened, this effort was dropped. If it hadn't happened, then charging users would have expanded gradually over time.