From one side all of socialmedia mechanics were tested before it's launch, from the other the market was already divided and not too easy to jump in. Google went all in with soccialmedia madness and topped AlexA ranking thanks to G+.
G+ launched with several techniques that allowed mass growth natural and forced. The first were circle sharing and circle following: You were able to gather people by the criteria like #socialmedia for example then share them as a circle and another person could add them in bulk up to account following limits. The second were suggestions to join to G+ to all the Gmail accounts and conversion of YouTube and Blogger accounts to G+. It was unique ripples visualization that was pretty useful in watching and refining viral techniques. In addition Google made G+ pluses and posts matter much in the search and it's exploded :) SEO and SMM professionals started to use it powerful tool by hand and guess automated and broken ecosystem. I can't say the network has specific audience of webmasters and it's extremely effective in driving traffic to Google friendly resources still.