This article from Mashable.com is probably one of the best, simplest primers on how small organizations can use social media. And while it's written for small businesses, it applies to nonprofits too.
Every sentence rings true for me, and I've heard myself saying most of them to most of my clients. Smaller organizations often feel like they "need" to do something with social media, but rarely appreciate the appropriate tone to take and how much work it can actually be to have a consistent social media presence. But as you build your tribe, social media will be one of the most important modes of communication and interaction, so it's worth the time and effort to hone this skill.