8 companies from the Y Combinator accelerator programme are now worth more than $1bn

"Total "valuation" of all YC companies: >$65 billion
Total money raised by all YC companies: >$7 billion
Number of YC companies worth more than $1 billion: 8 [1]
Number of YC companies worth more than $100 million: >40
Number of companies funded by YC so far: ~940
Number of companies funded by YC that have dissolved: 177
Number of companies in the last batch: 107
Number of hardware + biotech + healthcare companies in the last batch: 32
Number of companies we offered to fund yesterday for the first YC Fellowship: 33
[1] This includes Twitch, which Amazon bought for ~970MM plus an earn-out."

Source:  Blog post from y Combinator, 26th August 2015