I am not using Facebook for the last 2-3 months and I must say, it is a positive change in my live.
Nevertheless, I would argue that the gain isn't measured in time - I think it's about the quality of the time spent.
We need free time - its the best way to "always work by choice", isn't it? Having the "power" to start and stop whenever you want, is a smart trick to make your work a lot more satisfactory as a process. Maybe if I had this perfect discipline in me and I didn't need such psychological tricks to help me, I could be as efficient without any free time what so ever, who knows ...
Yet as a regular human being, I need to feel free to work or not to.
But to make the most of my free time I've decided to distil the choices how to spend it down to a "better" list, so I don't need to try ripping this "pleasure" out of my schedule.
So I read random stuff, but on the top pages of Hacker News, I read random stuff but in my not-so-randomly selected list of blogs that I've subscribed or twitter feed, etc.
At the end, it's the same time I am away from my work, but the quality of that time, measured as helpful information I've digested is substantially higher, at the same (often higher) level of delight.