Why is Google+ gaining less traction and is less popular than Twitter and Facebook?
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When Google launched G+, they were trying to compete with Facebook and Twitter, which they soon realized was nothing more than a fantasy despite the amount of funds they pooled into it, and they carved their own path with it. There are significantly less users on it than FB and Twitter, but one of its best perks is that it can be used as a tool to increase your page rank. If you attach it to an article, for example, your PR for that article can raise significantly the more people on Google+ like or link to it.
As for why it is gaining less traction is because the concept isn't very unique or interesting. It's not enough to pull people away from Facebook and Twitter, and until something more ingenious than the two comes along, we won't be seeing that happen. If anything, Google+ is geared more toward webmasters.
I think it might come down to the fact that Facebook and Twitter had a BIG head start, and I don't know any famous people who have been shouting about Google+ like they have with Facebook and Twitter, maybe Google could stick their hands into their pockets and pay Justin Bieber to rave about it.