Google failed in part because they never really communicated what Google+ is. I still don't really grok what it's for or why I should use it, and that's why I don't. They really just didn't produce a compelling product, brand it well, or market it effectively. For whatever reason, Google's marketers doesn't appear to be of the same caliber as their engineers.
Someone will pull Twitter's pants down and at the same time eat their lunch. I love Twitter, but I can't wait to see what cool thing replaces them.