This analysis is so good it's worth quoting from not just retweeting.
But a social network isn’t a product; it’s a place. Like a bar or a club, a social network needs a critical mass of people to be successful—the more people it attracts, the more people it attracts. Google couldn’t have possibly built every one of Facebook’s features into its new service when it launched, but to make up for its deficits, it ought to have let users experiment more freely with the site. That freewheeling attitude is precisely how Twitter—the only other social network to successfully take on Facebook in the last few years—got so big. When Twitter users invented ways to reply to one another or echo other people’s tweets, the service didn’t stop them—it embraced and extended their creativity. This attitude marked Twitter as a place whose hosts appreciated its users, and that attitude—and all the fun people were having—pushed people to stick with the site despite its many flaws (Twitter’s frequent downtime, for example). Google+, by contrast, never managed to translate its initial surge into lasting enthusiasm. And for that reason, it’s surely doomed.
