It's a great company doing great products.
You say that you realized doing a startup was not doing what you love. I fear you are unconsciously doing the same mistake - doing something you don't love, because in your own words:
"I'll be honest: I realize that Zynga is no Charity: Water. I'm not going to be improving someone's everyday life like Dropbox (students, teams, everyone), Instacart (everyone), Amicus (nonprofits), or Gumroad (content creators). But ..." no buts.
Social gaming may be less ambitious but they do it perfectly and that alone makes it a worthy goal.
Love it or leave it.
EDIT: with your reply I think I now better understand your move. I was just worried- we sometimes do the same mistakes. I'm very grateful to friends who have avoided me doing the same mistake too many times :-)