From 12 Awesome Social Media Facts and Statistics for 2013 @ jeffbullas.com:
Facebook still dominates at 70% of account ownership but Google+ is not far behind at just over 50%. Keep in mind though that Google+ account is mandatory whenever you create a new Gmail account. This is pushing up the account ownership stats. No other social network has Google’s web assets leverage.
The large Chinese internet user population is producing some large Chinese centric social networks including Sina Weibo, Tencent Weibo and Qzone. So Facebook doesn’t just have Google+ breathing down its neck but the rise of China’s social networks will possibly be a threat in the future.