Peter, the same sorts of things ring true for me and I'm doing things to try and improve my life; joining some clubs (just being more social in general), changing jobs, training myself up in my chosen career path, changing my diet dramatically and just spending more time with my family. I've had similar depression bouts on and off for a couple of years and just decided that it was time to change. Those same principles can help you too, just think about what it is that you want to do and plan out a way to do what you want and where you want to be. Also join some social clubs, whether they be meetup groups in your local area or active groups like martial arts, sports or recreational activities like drama/acting or musician groups or even something as obscure as learning archery. Maybe trying something new and thinking outside the box will help you to do something you would never had imagined you would enjoy. It's time that you made some new friends. I also quit Facebook a couple of years ago and it was definitely for the best. I find that the more you sit around and think about these things such as "what have I been doing with my life?" the more depressed you will be about it, so try and get out and do these things asap.
There's more people that have been in your position than you may realise (such as myself) and as shown by people on here, we're here for you through these difficult times. You can chat with me anytime via PM.