As previously stated, many distributors will have marketing procedures in place to help sell your film when it is ready for distribution. The true use of a social networking strategy comes long before your film is ready for distribution.
A social networking strategy will take many months to a year to implement and it will be an ongoing effort. This effort starts with you and your team first. You will determine whom you are trying to attract into your community and what you have to offer them of interest. It is NOT all about your film, in fact very little direct mention of your film is best. Follow the 80/20 rule, 80% of your assets are about the interests of your audience, 20% of your assets are telling them about the film. You will build your engagement pages and populate them with interesting and valuable content. You will not be asking your supporters for ANYTHING, merely building a solid base of supportive fans who will be there when you are ready for distribution.
You should never do anything that will make them feel that you have formed the community in order to use it for your own purposes. Companies and filmmakers who do this stand to ruin the very thing they have spent so much time developing; a genuine and authentic community that is very loyal and connected to you and your film. That kind of loyalty is extremely difficult to accomplish with advertising and it is really the ultimate goal of all brands.