The Facebook-Zynga relationship has become deeply unhealthy. Shares of Zynga were falling through the floor in mid-morning trading today after it posted disastrous second-quarter earnings Wednesday. The online game developer was hit hard by tweaks Facebook made to its platform that favor newer games over Zynga’s established brands like FarmVille and CityVille. Now, Zynga is returning the favor by turning into a significant drag on Facebook shares ahead of the social network’s first quarterly earnings report as a public company later today. Time for couples’ therapy?