The biggest boneheaded decision from my perspective was their taking over the + prefix in Google search (to filter for results that have this term verbatim). That just positioned G+ as my enemy and I had a strong desire for it to die. Unfortunately, they didn't bring back the prefix even after it died. Quotes around a term do something similar, but I am still angry.
According to Wikipedia, G+ usage kept growing from about 40M that October to 90M by the end of 2011 and then to hundreds of millions over the next few years, but the reporting methodology seems very inconsistent.
