Facebook
…. is a social networking service that lets you connect with friends, co-workers, and others who share similar interests or who have common backgrounds. Facebook have some block and some of them is:

1. Profile – A profile is the online representation of a Facebook entity.
2. Friend – Friends, of course, are what social networks are all about. A user has a list of friends, who in turn have attributes. Usually a Facebook application operates on one’s friends and their attributes, such as interests or movies they like.
3. Page – Usually a page is a frame.
4. Group – A group is made up of people. Usually a group has some overarching goal and activities associated with it.
5. Photos – Image data is uploaded to Facebook and kept in albums. Photos can have tags.
6. News feeds & events – News feeds are notifications about some activity, for example adding a comment about a photo or a new video. Facebook applications can organize events and reflect them as feeds.
7. Requests & invitations – Requests are initiated from applications. For example, if someone wants you to be a friend, it is called a friend request, some applications might want you to join a group which is called an invitation.’The difference between a News feed and a request is that a request needs the user to do something, such as authorize permission for the application or grant a person to become one’s friend while a News feed is a notification.
8. Wall – A wall essentially is a frame on which friends can post messages.
That is for me the most important when I talk about facebook, if you won`t to read more look at newbie

LinkedIn
LinkedIn is a social site where corporate folks feel at home. With the tagline “LinkedIn: Relationships Matter,” and characterized as “a networking tool to find connections and recommend job candidates, industry experts, and business partners,” it certainly couldn’t miss the corporate world. In fact, it has around 25 million users from more than 100 industries. Remember LinkedIn is professional social network and more about linkedin read at cnet

Business use of social networking is fuelling a revolution in collaboration so that don`t forget used it. More about trends, information age and social networking within the enterpise read at information-age

Microblogging, with Twitter as the foremost example, is a great social phenomenon that at times may be trivial but is certainly worth understanding and leveraging from an enterprise perspective. Twitter is very useful. For example, a group of friends can quickly organize a meeting in a pub or restaurant. It is an effective tool to quickly communicate with others. It has also become a personal news-wire medium for sharing world events (good and bad). One great post for twitter  said : “Twitter has not only tipped the tuna, but by some estimation, it has already jumped the shark.”
Read complete text Newbie quide to twitter