According to a recent research, Twitter users are forming tribes and frequently include keywords (hash) to communicate within the tribe:

Twitter users are forming 'tribes', each with their own language, according to a scientific analysis of millions of tweets.

The research on Twitter word usage throws up a pattern of behavior that seems to contradict the commonly held belief that users simply want to share everything with everyone. In fact, the findings point to a more precise use of social media where users frequently include keywords in their tweets so that they engage more effectively with other members of their community or tribe. Just like our ancestors we try to join communities based on our political interests, ethnicity, work and hobbies.