Ask HN: How safe is Google / Facebook sign in the long term?

Given that almost every useful websites allows you to sign on using Google Plus or facebook its easy to sign up for various sites. But what happens to your data once you stop using the third party site and revoke access from Google plus or facebook?

Looking to understand how safe is using Google/Facebook single sign on on site from a LONG TERM privacy perspective.

What are the advantages and disadvantages of using single sign on in the long term on a users privacy.