Facebook is in trouble. Not because of the questionable success of its IPO or difficulty finding ways to commodify its services (although, the latter may be significant contributor to the issue). Google+ is in trouble for the same reason, albeit at the opposite end of the spectrum. The reason Facebook is in trouble is because of the fact that it's signal-to-noise ratio is falling precipitously.
Back in the 1940s, Claude Shannon developed the idea of channel capacity as part of his Information Theory. He demonstrated that the effectiveness of a channel in conveying information was a function of it's bandwidth and, given that bandwidth, how much of it could you fill with signal as compared to noise. The more signal relative to the amount of noise, the greater the capacity of that channel.
I've been a Facebook user for a long time. I joined about a year after it launched publicly. As such, I've been party to quite a bit of growth. Early on, it was more of a curiosity. Most of my early Facebook friends were the geekier or hipper of my acquaintances. Then, as word got out, more and more people began joining and connecting. My family, college friends, even grade school classmates began turning up in my friends list. Back then, the platform was primarily about the status update authored by the actual user. There was some "friction" to the sharing, but the consumption was largely frictionless. I was interested in most of what I saw.
Then, they opened the platform to third-party developers and the game updates began. Suddenly, I began having to scroll through pages full of Zynga updates about my friends obtaining Neapolitan Cows in Farmville to get to the next content of substance. Invitations to join Mob Wars began to clutter my inbox. I was able to filter some of the unwanted content out by explicitly hiding it from my stream, but the friction had been passed from the poster to the reader. Personally, I'm in favor of a little editorial rigor. There's a reason newspapers don't just publish reporters notes as they take them; spending some time to figure out how to communicate what you're trying to communicate ensures that the reader's time is well spent.
The situation has only been worsened by Facebook's attempts to find new content streams it can commodify. Now, my wall is polluted with trending videos, articles my friends have glanced at, ads (sometimes hilariously incorrect in their demographic assessment of me), and interactions largely between people whom I don't know (because one of my friends liked the initial post). I have to scan quite a bit of content to turn up anything I'm actually interested in.
On the plus side, the signal is still strong on Facebook. That's where the people who can create the content that I am interested in (friends, family, colleagues, etc.) have set up social shop. Unfortunately, the content that my friends have explicitly chosen to share is being overwhelmed by content that they either didn't really chose to share or is being algorithmically chosen by the platform.
Google+ has very low noise. In fact, the whole concept of circles is designed to reduce noise. It puts a little more pressure on the poster to consider which of their circles would be interested in a particular piece of content, but the result is a content stream that is fairly pure and I am generally interested in much of what's posted in my stream. However, the signal strength on Google+ just isn't that great. There is a handful of people with whom I have connected that are always generating great content, but the diversity of content just isn't the same as id was during the golden age of Facebook.
In both cases, the issue is that either signal is not strong enough or the noise is just too loud for the user to be getting optimized channel capacity.