system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do social websites like Facebook, LinkedIn and Twitter know to suggest people that I haven't spoken with in 10 years or only had very brief dealings with a long time ago?
gpt
Friends of friends. Someone on your friend list may have recently spoken or friended one of those people. 

School association. Some of those people may have reported as being in the same high school or college as you.