It soft-launched in 2014, I believe. I registered on their website asking for an account sometime in 2014, and got an invite in September 2014 to what the email

It soft-launched in 2014, I believe. I registered on their website asking for an account sometime in 2014, and got an invite in September 2014 to what the email described as a "private beta". The general public launch where anyone can just click and create an account was earlier this year.