They've very cleverly used Foursquare as part of the promotion, but not in a conventional way.
What they've done is to check into various places around London and leave the following tip:
"ATTENTION: Exclusive footage has been found from the upcoming release of Monsters (Dec 3rd). View on mobile http://bit.ly/bL5whU or PC http://bit.ly/atYSgu – Follow STRA Agents for more information"
But... It's also pretty spammy. I'd hate it if lots of companies did this, and I'm sure Foursquare would act pretty quickly if the practice became widespread.
Update - it seems that it was done in partnership with Vue and Cineworld - special 'infected' zones have been set up.
Update - they're trying to organise Foursquare swarms (i.e. more than 50 checkins at once) at a number of venues on launch day, 3rd December. If a swarm happens, all participants get a discounted ticket:
"On the 3rd December (release day) from 6pm, fans who are eagerly awaiting the release of Monsters are encouraged to gather in 5 Vue Entertainment cinemas and 5 Cineworld venues around the UK. Using the Foursquare application on their smart phones, fans will need to unlock the coveted Swarm Badge, which can only be unlocked when 50+ Foursquare users check in to a venue simultaneously.
Unlocking the Swarm Badge will unleash a discount to see the film that very evening at Vue Cinemas (7pm) and Cineworld (TBC) in the following participating venues: Vue Birmingham City Star, Vue Bristol Cribs, Vue Cheshire Oaks, Vue Leeds Light, Vue Westfield, Cineworld Birmingham Broad St, Cineworld Cardiff, Cineworld Edinburgh, Cineworld Greenwich (O2) and Cineworld Sheffield. "