I think "monoculture" or just centralization in general would be a better name than "monopoly" for what the OP is getting at. Hacking is all about decentralization to ensure free access. If github implements a policy (surely there's some action permitted by TOS, perhaps which most people would like, that you wouldn't), UI change, etc., you have no real recourse outside of convincing them it's in their best interest to change it to how you like it. With a decentralized system where you run your own node you can modify the code and behavior at your discretion.

On top of the possibility for customization and extension, having more independent nodes means less chance of catastrophic failure. The cloud is remarkably unstable; look at heroku's uptime and at cascading failures even in redundant cloud systems for an example. It'd take the whole internet falling apart for thousands of individual hackers' repositories to all break down.

I don't disagree, but of course centralization is part of what has made github so successful.

There is always this thing about centralization. It makes initial social networks easier to form and the value comes from that.

And then we would like to see people eventually move to decentralized tools doing the same thing. But so far this second phase hasn't been easy. The reason is people building installing these decentralized tools.

We've spent the last 2 years building a platform that will make building and deploying decentralized apps in Node.js easy :)

A platform where one of your friends hosts something you like, you click a button and download and install it from them. Versions propagate throughout the system. The security is a key component -- you need to make sure it is signed by the developer. Also you can decentralize the app stores and centers of trust. If some of them are reporting the software as malicious, you will know.

That last part is hard. You are hosting the software on YOUR computer, so malicious software may be dangerous. It either has to run in a sandbox (as a website does in a browser) or it has to be proven to be safe by someone.

The same way Windows and Office are successful?

No, the way Facebook and eBay are successful.

While I agree that Bitbucket and Google Code are interesting competitors, I think the "popularity" of SourceForge is misleading.

SourceForge is only big because they are already hosting so many projects. I haven't seen any intresting new projects there. And those being there are would mostly be willing to switch to GitHub, if they had enough spare time for that (it's not just the project switch, but also the switch from CVS or Subversion to Git they would have to manage).

GitHub's social features don't extend outside GitHub, though.