They said they're under a DDoS attack https://twitter.com/github/statuses/259029493669310464
OTOH, if I had a botnet and I wanted to see how powerfully it could DDOS without drawing a lot of mainstream media attention, github might be good for target practice. They have better infrastructure than most, and they always do detailed write-ups afterwards.
I sort of expect it wouldn't be an effective practice anywhere.
Honestly, grow a pair, fellas. It's part of doing business on the Internet.