Interesting to see that now the current stable versions of Chrome and Firefox have dropped spdy/2, which is the only version of spdy supported by the current stable version of nginx. And nginx is the server used by more than 90% of hosts supporting spdy[1]. I presume this means it is mostly a Google/Twitter/Cloudflare thing for now since they seem to all be running spdy/3.1 in production.
[1] http://w3techs.com/technologies/segmentation/ce-spdy/web_ser...