Communities in the traditional sense are a group of people who reside in the same area, share a common culture, heritage, interest or characteristics and perceived as distinct from the larger populous within which it exists. (Dictionary.com)
Today thanks to the internet and social media virtual communities are proliferating and the members of a community no longer need to reside in the same area. Social networking enables larger online communities which help people learn more about entities in the network. It has become very easy to build a community using one of many social networks. It would seem the only limitations are access to a social media network and any country restrictions of social networks- I expect the communities will continue to grow as the world becomes smaller as a result of social media.
There are many interest based communities which have been around for a while and which have evolved to meet the needs and desires of the participants. LinkedIn has many groups which allow people to connect with like minded individuals for many reasons; to learn about each other and industry trends (such as BICSI and Structured Cabling Professionals), pose questions and receive answers from the members; find local and virtual training partners (Executive Athletes and Triathlete), to connect with individuals they worked with at past companies (Ex-company groups). Some communities are open allowing anyone to participate while others are closed requiring individuals to be accepted into the group.
Facebook allows users to create groups (communities) for many purposes; to keep others easily informed of upcoming events and past happenings and to track challenges. One the groups I belong to the North Atlantic Audi Group (NAAC) promotes the clubs upcoming events and allows others to post Audi, club and other car related comments for discussion.