I don't have any experience with Facebook/MySpace (programming or even really spending time on their sites), but...

How do you want to contact people? Email, SMS, IM, phone, nearby hackable electronic billboard *grin*...the more options you provide the better your coverage. (Some of us use SMS only as a last resort.)

Geo-based filtering might be useful so that a call to arms for a protest in Podunk doesn't annoy the activists in Artemisia.

Give people an easy way to invite others along who haven't signed up for your alerts.