Backstory: We recently went to a local beer festival and I happened to jump on instagram right before we entered. I noticed a friend posted a picture and he was at the same festival. I quickly shot him a text and we met up and hung out most of the day. If didn't notice his post on instagram I wouldn't have known he was there.
The Idea: A simple notification when you are near a friend.
Deep Dive: This would be useful for events and weekend nights so you can meet up with friends that you didn't coordinate with ahead of time. This is most useful for friends or acquaintances that you wouldn't normally call but still enjoy their company.
You could geofence certain areas and restrict the times/days when you want to be notified. There's a growing trend for people to move away from facebook, this is one of those niche situations that would be perfect to offer.
Validation: I already sift through facebook when I'm at a baseball game to see if other friends are at the same game.
Monetization: This would be hard to monetize, as it's a relatively simple B2C app. You would have to run ads to recoup and costs.