Personally I find my direct usage of some mature social networks decreasing for the reasons that Mike and I outlined.
You can unfriend people. You can unfollow. You can unsubscribe. You can hide/block/remove almost everything you don't like.
I get your point that human nature makes it a lot harder to do these things, but the tools are there. Every time I meet people who complain about some social services I tell them that the services are exactly what you make them, and not the other way around. They are so broad, so general, and so open that you can make your experience exactly as you want it.
And that's important to remember. If Facebook sucks for you, it's mostly because YOU made it suck. :)