Analyzing Twitter Relationships
My metric of choice would be the communication by direct message. However, this metric is hard to come by since measuring it requires the user's consent.

I do not think that using similar hashtags or terms has any say over friendship. People tend to adjust to the language they read and do not require personal bonding to do so.

I think mentions are a good way to identify friends and foes alike. A good way would probably be to see whether mentions beget a reply. Users tend to 'reward' pleasant and useful mentions with replies. However, the semantics of these messages are very important and only quantified analysis may not be enough.

Since I cannot comment yet, I would like to reply to the suggestion of using machine learning. I do not think that machine learning can effectively identify friends, even if one manages to get 'sufficient' data. Using Twitter is a rather creative process and generally not uniform across social circles.