All throughout 2015, the amount of http2 traffic has been increasing. In early September, Firefox 40 usage was at 13% out of all HTTP traffic and 27% out of all HTTPS traffic, while Google sees roughly 18% of incoming request as HTTP/2. It should be noted that Google runs other new protocol experiments as well (see QUIC in 12.1) which makes the http2 usage levels lower than it could otherwise be.