To talk with all kinds of people
To stalk all kinds of people
To try to get a date/relationship/hook up
To keep in touch with your friends who have moved away
To troll, cyberbully, or otherwise intentionally make people feel bad
Because someone told you to sign up, and you have no free will
Because everyone else is doing it
To keep track of or host events
To make friends online due to the fact that you have no friends outside of the Internet or social media
Because you're Mark Zuckerberg
To look at pictures of cute animals
To stare at pictures of hot people
To find out what happened to your old classmates
To show off your body in your profile pic
Because Twitter, tumblr, and Grinder just don't do it for you
To use it in a way that makes you money
To use it for a cause, organization, or other group
To answer stupid polls like this one
To meet people from all around the world
To post clever status updates
Because Twitter doesn't allow you to post selfies
To have an online relationship with someone
Because you don't want to feel old anymore