Tracking you down, even if they don’t know your name
Can't remember the name of the guy or gal you, uh, connected with at that random party? No problem. Facebook's creepily powerful Graph Search, the semantic search engine Zuck & Co. rolled out in 2013, allows people to search content on the entire social network using natural language. The tool has been scaled back significantly since then and no longer exists in the way it once did, but it can still be used to reveal quite a bit of information about someone.
Let's say all you remember about this person is where they work or went to college (or both). Search for "people who work at X company" or "people who went to X college." Combine the two for even more narrowed results. Maybe you met them through a friend who works at Thrillist. Search "friends of people who work at Thrillist."
There are literally hundreds of ways to narrow people down. With some simple sleuthing and a little patience, people can find you based on even the smallest detail.
Seeing where you've been...
There was a time when Graph Search would allow you to easily find out what parties someone's been to, or locations they've checked into. And while you can no longer find those things out by submitting queries like "places X person has been in the past X months/years" or "events X person has attended," there are third-party websites like this one that will easily uncover and organize that sort of information for you.
... and what your interests are
Unless you've Fort Knoxed your privacy settings, anyone can easily find out the bands, movies, books, websites, people, or groups you like using the aforementioned stalking site.