I don't know the exact percentage of properties not in Zillow but in the MLS, but it is a very small amount. Zillow changed the game when it came to us non-Realtors(R) having access to housing price data. So much so, that some housing economist types have noted what appear to be permanent changes in months of active inventory (eg https://www.piggington.com, who analyzes San Diego's market)

I don't think it's hyperbole to say Zillow has more data on residential real estate than any other company in the US.