SSN alone is not very useful unless you have access to a government or some corporate database, but those are kept very tightly locked up, and they can be searched by name as well, so having an SSN just makes it a bit faster to find the right person.

When combined with other info, SSN is the final confirmation that lets you apply for loans in the name of the person, or log into their banking website. And the "other info" is a lot easier to get than SSN: Name and birthday from Facebook or other social media, address and phone number from white pages or many sites that copy that info or hack for it.

This is why you should never put your actual birthday into social media or any website not related to major offline transactions. The only organizations that need to know your true birthday are banks, work, school, landlords and doctors. For everybody else, pick Jan-1 of your birth year, or a year that's before or after; if you want the correct Zodiac sign, change the day.

Likewise, no social/entertainment website needs the full & correct spelling of your name, you only need it when you pay with a credit card, and they will ask for it separately