I agree - it depends what you hope to achieve. If you mean advertising and marketing outside the realm of social media sites themselves, then treat it like you would marketing for any website.
Marketing within social networks themselves is another story. If it's just volumes of users and getting lots of users signing up to your social media profile as friends, then you can do it the cheap but time intensive way - search for similarly themed social media profiles as yours, take a look at their 'friends' list and email / post a message on their wall asking if they'd like to take a look or become a friend of your profile. Be transparent about who you are (an employee of a charitable organisation) at all times and this should work over time. If you work hard enough to develop lasting relationships with certain users, they can become brand ambassadors of a sort and promote your profile to their friends and so on. All obvious stuff, but effective. There are a whole load of other approaches which we manage on behalf of our clients (owners of youth brands). There's also advertising on a social network's own ad platform which isn't overly expensive.
I'd be happy to discuss this further offsite and explore ways in which we could help drive the campaign.