my hypothesis is that (a) no one likes "noise" (b) everyone has a different notion of what "noise" is and (c) these systems are designed to amplify noise (but there are ways to create noise dampeners).
Personally I find my direct usage of some mature social networks decreasing for the reasons that Mike and I outlined.
But it's also important to know that the tools are there in all service sto decrease the noise and increase the signal.
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. :)
I would disagree with you on this part. Noise is the social. The lack of "noise" is what made G+ sooo boring for so many. I know once I added the noise of HN circle on G+, even though I didn't know any of these people, I enjoyed G+ tremendously more.
Personally I find my direct usage of some mature social networks decreasing for the reasons that Mike and I outlined.