Github, if they're honest about their 12 month uptime levels would be lucky to be a single 9 service. Their uptime is Terrible with a capital T. But you know what? Until there's something better everyone is going to keep using them, right?
Great services with values that are hard to find become damn near irreplaceable even with terrible uptime. This is an obvious place to compete; if you made a github clone that simply stayed online you could win market share during every downtime. However cloning github would not be trivial.
And therein lies the problem and the answer to why we accept their terrible uptime levels. They give us something we can't get elsewhere: social coding and easy centralization.