While I can't give you examples of companies using github for hosting, I can suggest some other arguments you could use in favour of using it.
One of the major advantages of distributed version control like git or mercurial is that each repository is a full or almost full backup. So you'll have central server hosted and managed by someone else, but even if anything catastrophic happens and github goes out of business, the damage will be quite limited.
You can also note that administrator you'd otherwise have will certainly be less experienced than the ones in big hosting company like github and thus your own server is actually more likely to fail. Because in a big server farm, individual servers die now and than, so all recovery procedures are tested. While with own server it will work fine for some years, so when it fails you can easily find yourself with broken backup or missing bits or such.