If you have 500 friends, each with a feed with 50 posts, you are talking about 25000 possible emails. Those emails would never be sent, because they are usually individually way below the importance threshold for an email.
So, by this calculation at least, it's at least a 1-2 order of magnitude reduction in the work required to maintain contact with old friends. That's valuable.
When G+ came out I dropped my Facebook account.
Due to the low activity on G+, I resorted to... doing stuff with other people in real life, such as volunteering and going to meetups.com meetups. And my real friends not near me kept in touch via e-mail and IM.
I went to high school in one city, college in one 1,000 miles away. Have since worked in cities all over the world. I don't know how I would manage to keep even a tenuous grasp on all the connections I have with people in these different places without Facebook. Admittedly, these are not and never will be deep social bonds I am maintaing, but that is hardly the point. There is value in maintaining a large web of shallow social connections. Facebook helps me do this.
Moreover, whether or not Facebook has improved the life of me or anyone I care about is irrelevant to an objective evaluation of the site's value. I could make the same case for anti-malaria drugs.
That is, the connections may be shallow while we're far away, but when locations change again, its nice to be able to re-develop those shallow connections into meaningful ones.
If you think there's no value in maintaining shallow social connections, then I think you too need to put some energy into adding meaning to your social connections.