Examples of services that typically would have been written in C/C++ but are written in Go: Docker, YouTube Vitess (a MySQL proxy), CockroachDB (a distributed and strongly consistent SQL database).
Other examples of companies using Go for network services: Dropbox (most backend), CloudFlare (Railgun and other things), Uber (geofencing), Bitly (messaging with NSQ), Disqus (realtime commenting), DigitalOcean, Heroku, SoundCloud, Dailymotion.
Other examples of companies using Go for network services: Dropbox (most backend), CloudFlare (Railgun and other things), Uber (geofencing), Bitly (messaging with NSQ), Disqus (realtime commenting), DigitalOcean, Heroku, SoundCloud, Dailymotion.