We do have access to records from the before times when the internet was wide open and Facebook, Google and Microsoft were big in China. Well, Microsoft is still big because they're not an internet company and unfazed by censorship, but the exit of Google and Facebook took a lot of pressure off Baidu and the entire Chinese social media ecosystem.
Facebook and Google were never big in China, not even close.
Google had 31% market share in 2010 http://news.bbc.co.uk/2/hi/business/8455712.stm I haven't been able to find numbers for Facebook.
Don't have a source either but I was living in China back then and basically no one was using it. It was QQ and Renren.