Doesn't it follow, regardless of causality, that if Facebook were just as good as face-to-face social interaction, then there would be no difference in happiness levels between people who use Facebook and those who don't? If Facebook had a positive impact on people's lives, then there would be a similar positive correlation in using Facebook compared to seeing someone in person.
Maybe there isn't enough proof to say "Facebook is toxic", but it certainly seems true that Facebook is not as good as real human interaction.