Aaron,
Please do continue to post about your progress and internal discussions about the value / risk of the social networking sites and the Mission of your YMCA. I think there could be value to the local community with some of the social network environment, however the reputation of the bigger social networks and the responsibility an individual has to exercise on their personal pages so as not to impact their career opportunities, can leave a confusing message if we use the same tool.
A story related by an executive about a phone interview with a job candidate, the exec told the candidate that their resume look great and related all the mission focus of the org. but he wanted to know how the candidate related the story told on the resume compared to the information on their social network page. The candidate hung up the phone on the interview.
As part of our employment process we look for social networking listings and see what is public and if they are saying anything about their employer etc..
An alternative you may consider is to set up your own social network with a private provider like http://www.ning.com/ this would let you control your "friends" and your environment more than the public sites, the down side is your members would have to maintain ther pages in your world and their social network world.
We've had some discussion here, unfortunately you have the age gap issues of getting the people over 40 to under stand the concepts of social networking let alone the risks.
Dave