Word of mouth on social media is all that is really needed.
You don't think that's
natural word of mouth on social media do you? Or that suddenly everyone who's anyone in entertainment is talking the latest Star Wars movie, when the franchise was (and frankly still is) nerd central? All that needs to be paid for, and Hollywood has a very finely tuned machine to drive chatter in the most seemingly-innocuous manner.
The Hollywood formula is even a little discernible these days - teasers on Youtube (aka trailers for a trailer), mini-trailer, TV spots (again trailers for a trailer), Trailer 1, sometimes Trailer 2 or Final Trailer, in between tweets, production stills, "leaks" of plot, images, script, actor interviews, posters, the making of, chat shows, soc-med popups and banners, all accompanied by articles and reactions to articles by the right people in the right places at the right time.
All that stuff operates on a defined schedule, all by design. Most of it unfolds as choreographed as Desert Storm, with some held in reserve and pushed out when the monitors see that views are flagging a little - time to push out some new juicy soundbite and obligatory OMG I CAN'T BELIEVE IT reaction tweets....aka "pulsing" audience attention.
Apparently it costs from 50% to 100% again the production budget, and yes theaters take 50% so a blockbuster movie usually has to make two to three times its production budget to make a profit.
And from a corporate point of view its 'necessary', because it was proven statistically a while back that the more marketing dollars are spent, the higher chance a film has of being a box-office success. There was even a correlation study IIRC linking number of trailers on Youtube and box-office. Ever since then no studio has said no to pouring on the marketing dollars for their big-budget hits.