Facebook for Strangers - find others nearby
A forum/Fb-type facility which allows people to find like-minded people nearby. Imagine a dating site, but for platonic relationships.
For example - add your postcode and your hobbies and it will show you clubs and individuals who share interests nearby.
Combine with local venues (pubs, coffee shops etc) to provide a "safe" venue for new friends to meet.
Combine with an Events calendar to encourage people to join in activities.