They kinda did. Before facebook brought them, the app cost $1/£1 per year (iirc your first year was free). Thing is back then MMS and/or texts across borders was expensive, so if you were regularly sending picture messages to people the $1/£1 sub was a no brainer.

Lets wave a magic wand and presume 50% of the user base thought it was also worth $1 a year and it grew just as well as it did (It was growing very well in the UK before the takeover just by word of mouth). That's still just a messaging app that would be raking in $1.5B per year today, and that's before you bolt on any paid cosmetics or upgrades (small things that users don't mind dropping a few more bucks on).