The player makes their own fun by setting themselves goals and challenges that match their personalities and skill sets (that they find enjoyable). As their skills develop (or they simply get bored) they naturally find new, different, more challenging things to do until the game can no longer satisfy them.

Zynga provide a free framework and a whole load of extras to enable people to have fun. You can progress for free according to a number of pre-designed interpretations of how you can progress in the game, or you can short-cut for cash and do the bits you really enjoy, and you spend more as you explore different options. It practically screams "revenue stream".

Obviously it's all set up to make loads of cash, but I see it as enabling fun rather than a scam or "accomplishment lite".