Well, the advantages are
– your personal data are mined and then sold to 3rd party companies without your permission or knowledge so you can be spammed with personalised advertisment
– Potential employers can read about your private life and ruin any chances of you getting hired
– The mere existence of Facebook and Co. has prompted employers to demand that you give them access to your profile, and if you refuse to comply or to even open an account you ruin any chances of getting employed
– You open your whole life to government agencies that can use the data to ruin your life
– Your innermost secrets can be seen by anyone, family included, that can then ruin your relationships with your family, friends and acquaintances.

What is not to like?

I never sign up for anything that requires me to use my Facebook or another account. It is purely for data-mining reasons.

And what @ragingloli says.