You can check in things that shouldn't be checked in with any language/framework.
If you have done this, here's how to fix it: http://help.github.com/remove-sensitive-data/
test / rails_app_v3 /
test_app / config
In many other instances, things are not as the seem. For example, some of these results come from commits where the author is moving the token to an environment variable. For example: https://github.com/cimm/blathy/blob/2d3a9550d3a0be55db8e26a2...I certainly agree that we should all be security conscious, but I'm also a fan of keeping perspective. Things are bad, but let's keep the truth in mind too.
This is like people storing plain text passwords in publicly readable txt files on a server. It's not a problem with FTP, HTML, Apache (pick anything you'd like) it's a problem with people making poor decisions.
"Is storing your private key in a public repository a security concern?"
It's a parody of a security question. This is a needless distraction in an important discussion.
https://github.com/search?q=FB_SECRET&repo=&langOver...
Not really a "vulnerability" because you can't keep stupid people from giving out their secret key.