Facebook has patented a "System and method for managing information flow between members of an online social network." That's the official title for patent number 8,010,458. Of course, Facebook as well as other social networks has been using this technology for a long time now, although Palo Alto's is arguably the most complex.
The inventors are Nicholas Galbreath of San Francisco, California, and Christopher Lunt of Mountain View, California. Facebook filed for the patent on May 26, 2004. It was granted on August 30, 2011 (today). Here's the abstract:
An online social network is provided in which members of the online social network control who may view their personal information and who may communicate with them. The members control who may view their personal information by setting a visibility preference. A member may not view another member's full personal profile if the measure of relatedness between the two is greater than the visibility preference of the other member. The members also control who may communicate with them by setting a contactability preference. A member may not communicate with another member if the measure of relatedness between the two is greater than the contactability preference of the other member.
In summary, this patent describes whether you can view a given person's user profile and whether you can view his or her full profile or a mini version of it. If Facebook decides to fight its competitors with this patent, it could be difficult for other social networks to offer the basic feature.
Google and Twitter are more or less safe because they have specifically chosen to offer public-only profiles. Other services, such as LinkedIn, could have a bit more trouble.
Here is a bit more detail. Facebook is making the following four claims with this patent:
The first claim is the key: it describes a social network where each member has sets of personal information. Someone interested in being able to see one such set for one such member must be closely related to the member in question. In this way, you may be able to see a set of personal information for one member but not for another. Similarly, the social network can permit messages to be sent between two members if they are closely enough related as well as forbid them if the relationship is too distant.
The other three claims simply describe enhanced functions for the first major claim. They outline making a mini-profile available to users who aren't related, providing a message interface so that two users closely enough related can send messages to each other, plus also cover e-mails and texts, respectively.
Again, Facebook isn't the only social network to use such a system, and we'll soon find out if it will be the first to enforce a patent for it. Technology companies often patent their implementations for defensive purposes: they don't plan on suing, but at the same time don't want to get sued themselves.