Google knows who you are on Twitter if you tell it:

https://plus.google.com/connectedaccounts

And because Twitter is largely public, Google knows who you follow and will show tweets and links in serps. Has done for some time now.

You can tell Google who you are on github, but I haven't noticed any interesting search results.

You right about password protected content though. You can tell Google who you are on Facebook but it can't crawl most of the data.