each level says "use 9 gold bars" (about $1 in cost) OR invite 3 friends (the request option, which then has two categories: invite anyone or invite just candy crush friends to help you). so there is no puzzle to figure out to get to the next level, it's just that every 15 or 20 levels, it's a new world and to go, they make you spam your friends.

This is how people end up spending a bunch of money because they don't want to spam their friends but there is no way to move on. But recently they added a "wait 48 hours option" and if people wait 2 days, they can move forward.