Another really scary one could be 'My phone rang at 3 am. The caller ID said it was me.' It plays on the idea of the self being something other or unknown, and calls at that time are always spooky.
Another great one is 'I always thought my cat had a staring problem. She does.' This plays on the uneasiness of being stared at by your pet in a way that makes it seem more than just normal feline behavior. It gives the sense that there might be something more sinister about the cat's stare.
One great 'imdb two sentence horror stories' could be 'I woke up in the middle of the night. There was a figure standing at the foot of my bed with no face.' Another might be 'She heard a scratching at the window. When she looked out, a hand with long claws reached in.'
I think 'The clock stopped at midnight. Then I heard footsteps.' is quite scary. It gives this feeling of something ominous starting right at a spooky time.
They often play on our deepest fears. Like in 'I heard my name being called from the attic. I live alone.' This taps into the fear of the unknown and the idea that there could be something lurking in our own homes where we're supposed to feel safe. The short format also forces the horror to be concentrated and immediate.