In three words: free public accounts.
If you Google any of the following terms, there is a high probability that a link to Github will be on the first page of the SERPs:
- Ruby on Rails
- Diaspora (as in the Facebook competitor)
- html5-boilerplate (this might be a bit specific, but you get the idea)
- Homebrew (i.e. for OS X)
- Active Merchant
- jQuery (actually page 2 for me)
Granted, all of these terms are very developer-centric - but that’s the brilliance of their strategy. Their target market is developers. So in a virtuous circle, when they provide free accounts to open-source projects that naturally are very popular not only are they loosely affiliated with that project, but they get all of the organic search engine traffic for the terms associated with those projects. The people that find them, will typically be developers that will become paying customers of Github - and those developers are likely to put up public repos that generate organic traffic to github on all sorts of obscure long-tail keywords.
I always thought that it was because Github wanted to be affiliated with these high profile projects (i.e. have them put a link to github on their project page, etc.). It never occurred to me that the bigger benefit is the massive organic search engine traffic that they likely get as a result of this strategy.
Brilliant Github. Brilliant!
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