I'm not on GitHub because I code for my employers. That work doesn't get put out there like that.
I don't tweet much at all about the languages I code in or cool bits of code I just wrote or any of that. I'm one of those weirdos that tweets pictures of my lunch and funny crap my 4yr old says.
I'm "on" StackOverflow since I have a log in... but I rarely comment on stuff and have never posted a question. I probably should I suppose but I don't.
I don't think my candidate brief on Skills would be a very accurate indicator of my actual skills.