A like button, like option or recommend button is a feature in communication software such as social networking services, Internet forums, news websites and blogs where the user can express that he/she likes, enjoys or supports certain content. Internet services that feature like buttons usually display the quantity of users who liked each content, and may show a full or partial list of them. This is a quantitative alternative to other methods of expressing reaction to content, like writing a reply text.
Use on Facebook
The like button is a feature of social networking service Facebook, where users can like content such as status updates, comments, photos, links shared by friends, and advertisements. The feature was activated February 9, 2009. It is also a feature of the Facebook Platform that enables participating websites to display a button which enable sharing the site's content with friends. When a user clicks the Like button, the content appears in the News Feeds of that user's friends. The button also displays the number of users that liked each piece of content, and may show a full or partial list of those users. This feature may appear differently on mobile web applications. A "Like Box" also allows Facebook page owners to see how many users and which of their friends like the page.
A lawsuit was filed in Los Angeles in 2010 claiming the Facebook should not allow minors to "like" advertising. Facebook said the suit was "completely without merit."
Rembrandt Social Media has sued Facebook, claiming that the like button violates two patents granted to Joannes Jozef Everardus van Der Meer in 1998.
The Like button is one of Facebook's social plug-ins, which are for use on websites outside Facebook, a feature which launched April 21, 2010, as part of Facebook's Open Graph, an interface for integrating websites with Facebook's social graph. Speaking at Facebook's F8 developer conference on the day of the launch CEO Mark Zuckerberg said, "we are building a Web where the default is social."
Since then the feature has aroused scrutiny over privacy concerns because the social plug-ins allow Facebook to track visitors to participating sites across the web, even if those visitors do not click the Like button, are logged out of Facebook, or are not Facebook users at all. The Like button is implemented similarly to an advertising network, and as more sites participate, gives Facebook a vast amount of information about who visits which sites, and when. When loading a web page which has the Like button enabled, the user's browser connects to Facebook's servers, which record the URL being visited, and the visitor's IP address and Facebook ID (if logged in). In June 2010 Facebook said it anonymizes this information after three months, and does not sell or otherwise share that information. The ACLU of Northern California cautioned website operators to be careful about installing Like buttons because "they're potentially telling Facebook about everyone who visits their web site, every time that person visits their web site."
By September of that year over 350,000 sites were using the Like button.
In August 2011 the German state of Schleswig-Holstein said the button breached German data protection laws and that federal agencies must remove the buttons and similar social plug-ins from their websites. Canada's Privacy Commissioner had raised similar concerns in 2010.
Research shows that Facebook Likes profile can be automatically processed to infer intimate details about an individual, such as sexual orientation, political and religious views, race, substance use, intelligence, and personality. Effectively, individual views and preferences can be revealed even if they were not directly expressed or indicated by Liking an associated content. For example, user does not have to Like "Barack Obama", "Being Gay" or "Being Black" to be flagged with appropriate category with high accuracy.
Anyone with access to users' Likes, e.g. applications and websites connecting with user's profile, governmental institutions, or even one’s Facebook friends could use software to infer intimate details that an individual may not have intended to share. One can imagine situations in which such predictions, even if incorrect, could pose a threat to an individual’s well-being, freedom, or even life.
Vulnerability to attacks
In February 2013, legal action was brought against Facebook by patent-holding company Rembrandt Social Media. Rembrandt owns several patents taken out by Dutch programmer Joannes Jozef Everardus van Der Meer, who died in 2004. These include patents filed in 1998 relating to van Der Meer's fledgling social network Surfbook, including, according to legal papers filed by the patent holder, the ability for users to approve data using a "like" button.  
On August 2011, Google integrated a form of the Like button it calls the +1 button to its competing social networking site Google+. The social microblogging site Twitter also launched a "Follow" button around the same time.
The amount of “likes" on Facebook can serve as a measurement of interest and/or popularity in a particular brand, product or personality. Not only can a large amount of “likes” be influential on shaping reputations, but can also lead to increased exposure - such as appearing in the “Recommendations Feed” - the “like” has an advertisement-value in itself. This has led to companies specializing in selling “likes” from fake accounts, which can prove to be troublesome for the “like”-mesasurement’s credibility. If nobody believes that these measures have any power or truth to them, then businesses can’t benefit from the advertising and social media marketing becomes impossible. Facebook has stated that using purchased “likes” is not permitted on their social media platform, and has an automated process trying to eliminate “likes” gained by malware, deceived users purchased bulk “likes”.
Like buttons as used by social networks on websites other than their own are often used as web bugs to track user activities for targeted advertising such as behavioral targeting combined with personally identifiable information (PII) and may be considered a breach of browser security and internet safety privacy concerns.
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