I quit Facebook 4 months ago. The relationships that are important to me have improved(even long distance ones), my productivity is up and my depression is down (not sure how that works or if it's related).
There has been studies done which show that the 'keeping up with the Jones' aspect of facebook can increase depressive tendencies. So the improvement of your depressions could be as a result of this.
I encourage you all to give it a try.