4. SHARING INFORMATION.

We can define social networks as virtual environments for comunication, interaction and the sharing of resources.
4.1. Horizontal social networks.
The users of horizontal networks form communities who have diverse interests and a wide variety of reasons for using the application.
The most popular are:
Facebook: This application allows groups of 'friends' to publish both public and private messages with text, links, news, photos and videos.
Twitter: This social network has some unusual features.
Users publish short text messages (tweets), which is called microblogging. On Twitter, the relationships between users are not always bidirectional.
They use hashtags.
Google+: This social network allows users to created 'circles' of 'friends' in order to share information.
They can share photos and form public or private communities.