Often it's related to username re-use, but if someone who knows you runs across your account, or if someone has see other profiles that look like yours, they can use the personal details you share to narrow it down.
For example, you could put the pieces of my posting history together to learn that I'm a sysadmin for a large company, an outdoor-cat owning cis lesbian in my mid 30s, I have two younger sisters, my favorite artist is Taylor Swift, I like working on cars, I'm an Arch Linux user, and I live in Liberal Mountain, ID but sometimes visit family in Liberal, MO. That overall profile doesn't apply to a lot of people, so if you know me or you've put together profiles of other accounts on other social media sites that mostly line up, you can make a pretty good guess.
This is why it's a good idea to lie on the internet.
Often it's related to username re-use, but if someone who knows you runs across your account, or if someone has see other profiles that look like yours, they can use the personal details you share to narrow it down.
For example, you could put the pieces of my posting history together to learn that I'm a sysadmin for a large company, an outdoor-cat owning cis lesbian in my mid 30s, I have two younger sisters, my favorite artist is Taylor Swift, I like working on cars, I'm an Arch Linux user, and I live in Liberal Mountain, ID but sometimes visit family in Liberal, MO. That overall profile doesn't apply to a lot of people, so if you know me or you've put together profiles of other accounts on other social media sites that mostly line up, you can make a pretty good guess.
This is why it's a good idea to lie on the internet.