It's in the sequel, "Paranormal Activity 2," that we learn Micah's fate. He is attacked and presumably killed by the demonic entity during the events of the first film.
Here's what happens:
* In "Paranormal Activity 2," the footage from the first film is shown as home videos found by Katie's sister, Kristi.
* Throughout the second film, we see glimpses of what happened to Micah after the events of the first film, including his apparent death.
* The details are left ambiguous, but it is heavily implied that the entity killed Micah.
So, while Micah's death isn't directly shown in the first film, the events of the second film establish that he died sometime between the end of the first film and the beginning of the second.