Writers of quite a few synopsis/reviews/articles seem to believe that the malevolent entity in the films is a ghost, rather than a demon.
The Danza: Everyone in the first film. The characters Katie Featherston and Micah Sloat share their names with the actors playing them, while "Amber" is played by Amber Armstrong and "Dr. Fredrichs" is played by Mark Fredrichs.
The first movie went up against Saw VI, in a case of two subgenres of horror duking it out. Its resounding victory at the box office, despite playing in fewer theaters, is often regarded as the death knell for the Torture Porn genre that the Saw franchise served as the face of, precipitating the rise of supernatural horror in the '10s. The second film going up against, and beating, Saw 3D the following year merely finished the job.
The first film also dueled with The Fourth Kind, another found-footage, supernatural-leaning horror movie released in the fall of 2009 (even if the baddies in The Fourth Kind were aliens instead of ghosts or demons).
Enforced Method Acting: Much of the dialogue for the first film was unscripted, with Katie Featherston and Micah Sloat improvising most of it. Also, in some scenes, they weren't told when anything scary was about to happen in order to get a genuine reaction out of them.
Executive Meddling: Averted. When the rights to the film were initially purchased, the studio put the film on the shelf and intended to remake it with a larger budget and more famous actors in the roles; when audiences walked out of test screenings for the film because they were scared, the studio quickly reversed their stance.
Fans now call the demon Toby thanks to Kristi calling him as such in PA3.
Paranormal Activity: The Marked Ones, due to its mostly Hispanic cast, has been called Paranormal Activity: The Marked Juans.
Franchise Killer: The Ghost Dimension may have been intended as a Grand Finale, but even if it hadn't, its poor performance at the box office, stemming partly from a VOD distribution strategy that caused some theater chains to boycott the film, likely would've killed the series regardless.
Now there are recent channels on Facebook, Twitter, and Youtube that follows a man named Jacob Degloshi. The videos appear to be leading to Paranormal Activity 5 while giving more detail on the Featherston sisters' past.