Google has a social network, and it isn't mentioned in the piece. But I'd bet there's little doubt Google is the most important search engine.
You're right about Dribbble, but I'd argue GitHub is for collaboration and Dribbble is for showing off. Certainly people collaborate on Dribbble and people show off on GitHub, but by virtue of how people actually use the platforms I think GitHub is more collaborative and more important (at least in the way the OP posits). Go to http://dribbble.com/, click the first thumbnail, and count the compliments in the comments.
You're right about Dribbble, but I'd argue GitHub is for collaboration and Dribbble is for showing off. Certainly people collaborate on Dribbble and people show off on GitHub, but by virtue of how people actually use the platforms I think GitHub is more collaborative and more important (at least in the way the OP posits). Go to http://dribbble.com/, click the first thumbnail, and count the compliments in the comments.