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.