I'm not even sure if this is the right StackExchange to post this, but it seems like sentiment analysis would go here.
What would be the best approach to determine if two people on Twitter are actually friends?
I think that the first criteria would be that they are following each other. This would eliminate all of the celebrities, companies, etc. that a person follows.
The second criteria could be physical proximity to one another. If two people tweet from the same general areas, then there's probably a higher chance that they actually know each other.
After that, I'm not sure what criteria I should look for. Would sentiment analysis of their tweets to each other be a viable option? With that friendly sentiment could indicate that they actually know each other.