There might be a huge opening here for a "companies only" facebook clone, where you can only get updates from your favorite brands and service providers, not socialize with friends.
Isn't this just Twitter?
And while valuable in it's own right has no where near the engagement numbers of Facebook.
Twitter doesn't quite fit nsns's description as a 'Companies Only' social network, as much of it is social. I don't necessarily know that anyone would willingly sign up for a companies only social network, despite what companies may dream and pray for :-)
LinkedIn is mostly about connecting professionals to each other and companies
Facebook (in a business context), is mostly about connecting brands with consumers.
Consumers are not professionals, although the underlying people that participate in those roles may largely overlap (if you think about it as a Venn diagram)
The more a particular company or service (or musician for that matter) deviates from the original product that attracted its user base in the first place, the greater its susceptibility to clones becomes.
I think all people want is simple and private social networking among their friends.
Facebook no longer offers this. Google+ certainly never did. There is a strong case that Myspace fell because at the time, Facebook offered more simplicity and more privacy.
Myspace was being deluged by spam during the summer of 2008 and that marked the beginning of the end for them ( http://www.google.com/trends/explore#q=facebook%2C%20myspace... ) Facebook is becoming equally noisy, except they are getting paid for the "spam."
As Facebook continues to aggressively attempt to increase its monetization, both on desktop and mobile, I think its going to get worse.
There is clearly an opening for a competitor coming along. I'd like to think of them as the Dropbox of social networking.
But now, FB has strated breaking that model, differntiating between human friends and the other (paying) kind.