The web and movies have very similar target audiences, and Hollywood likes the demographics. Ages 12-45, who spend a lot of time and money on electronic equipment, well-educated, and financially stable. These market use the web for a reason - to find information and maybe even have a little fun, and this is what a movie site provides.
Numbers of people who are visiting movie sites are also increasing. Starship Troopers, for instance, was growing at a rate of more than 5,000 virtual troopers a day, even before its Nov.7 release.