How can someone stalk you on twitter or facebook? How can you tell?

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Someone could easily stalk you one Facebook or Twitter.
On Facebook, if you are friends with the said person or don't have privacy turned on, they could simply subscribe to your posts and check your wall for all of the actions you've performed while logged in.

On Twitter the said person just need to find your profile al scroll through it to see what you've been up too.

A good way to tell is if the said person all of a sudden seems to know a lot about you, They could bring up certain things that happened to you a while ago, such as asking you about a vacation you went on 3 years ago.