There were multiple mistakes other than that, but for example initially you needed an invite to Google+ and they artificially limited the number of invites. This worked great for them with gmail, creating an inflation of demand and a fomo effect but with Google+ it just meant if you were on Google+ noone you knew (who didn't actually work for Google) was on there and if you weren't on Google+ you couldn't get on unless you knew someone who worked for Google or was one step removed.

They built a business model that relies on network effects and then deliberately killed the network effect during the adoption phase.