At the time Facebook was starting to seriously suck and Google was still a trusted brand, but it failed mostly because of the invite system for the first few weeks. Google themselves closed the door to the possibility of huge migration.

You got an invite, but none of your facebook frieds did. It was an empty town so one naturally started adding random people, thus fucking the friend reccomendation algorythm forever. When they finally opened to the general public, people had lost interest already.

Their circles concept was actually awesome, but they shot themselves in the foot in trying to keep it exclusive for those first few weeks.