This may be obvious so forgive me if I'm teaching you to suck eggs.
Charities have great emotional appeal. They therefore lend themselves to social networking techniques that enable you to tap into the latent goodwill that most people have for good causes. The question is how to mobilise that into action and support?
You could start by setting up a Facebook group and by getting your staffers to promote it on their profiles. Then promote an event on the group and create a Justgiving page.
Otherwise in my experience charities, even some of the best known, are behind the times on search engine optimisation. One of my closest friends is a senior manager at one of the country's most high profile charities and when she showed me what their media agency were planning to do to promote their new website (as if a media agency would know about SEO - most don't), I was shocked.
Make sure you understand how people would be searching for what you offer/solve etc and that whoever manages your website has either the knowledge and experience to optimise it him/herself or a company with a proven track record.
Paul