Seybold Scientific had a great posting on what you need to do build an effective social media plan. It's written for businesses but is easily adaptable for film. Read the whole thing, but here's a brief on the first four:
1) Clearly articulate who your stakeholders are before you begin.
2) Clearly articulate the key issues these stakeholders care about as it relates to your offering. Use a bulleted list with no more than three or four words per item.
3) Begin by researching which, if any, top bloggers are discussing these issues.
4) Inevitably, any substantial subject matter area has a back channel where top bloggers and influencers chat.