On the State of Art of Decentralized Online Social Networks Olga Moreira December,2012 Niagara Falls, Ontario, Canada Abstract: Privacy breaches are the main source of apprehension in trusting centralized online social networks providers who often have absolute authority to control all of the users data. In last several years researchers and software developers have been testing decentralized social networks prototypes that can grant complete data ownership and security to individuals. The main challenge in decentralization is guaranteeing 24/7 availability without compromising reliability of the network. This paper overviews some of the methods currently used in decentralized systems to replicate data on trusted peers machines in order to provide its continually availability. 1. Introduction Presently the Internet is an essential part of our daily lives. Online Social Network (OSN) sites offer to individuals the opportunity to create their on space to share their knowledge online, to enhance their social allowing them to find new friends and to keep in touch with their acquaintances, friends and family across the globe. In 1997 SixDegrees.com launched and marked the beginning of OSN age. Unfortunately, SixDegrees.com did not attract enough members before failing as a viable business and shut down in 2000. Friendster launched in 2002 as a social complement to Ryze launched in 2001. Friendster rapidly gained popularity and grew to 3 million users in only few months . Its success inspired the creation of numerous OSNs from 2003 onward such as Myspace, LinkedIn, classmates.com, Tribe.net, Orkut, Hi5, Flickr, Facebook, Yahoo! 360, Youtube, Twitter, Google Plus+, etc. [Boyd2010]. OSNs as popular as Facebook and Twitter have attracted more than hundreds of million users over the years. Facebook was founded in 2004 and it became the world leader in social networking in 2008. Facebook has not stop growing, it reached 1 billion monthly active users as of October and 584 million daily active users on average in September 2012 [FB2012]. Because the success of OSNs depends on the number of users, they often race to improve their visibility on the Internet to attract new members while their security improves at a slower pace. As a consequence, third parties may access private information found in the profiles of OSNs users and use this information for devious purposes. For instance, early academic studies on threats to Facebook users privacy revealed a potential ability to retrieve social security numbers using information found in their profiles, such as birth date and hometown [Gross2005, Sattikar2010]. Services in the aforementioned OSNs are centralized allowing the provider to have the unique authority to control all the data of the users. Often users have to agree to certain policies when using these centralized OSN services which may involve: (1) the usage of their data for targeted advertising; (2) keeping users personal information after deactivating their account; (3) imposing restrictions on how new applications can be created holding back users creativity. The increasing distrust in centralized OSNs providers and the fear of privacy transgressions urged the industry and academia to investigate a decentralized solution to ensure OSNs users of having complete ownership of their data. 2. Decentralized Online Social Networks The search for alternatives to centralization of personal data began in the late 1990s, however, web community and academic computer scientists dismissed social networks until 2007 [Nara2012]. Open-source communities launched several social network in last few years based on decentralization principles, some examples are Diaspora, Friendica, OpenSocial, Noserub, Buddycloud etc, aimed to attract non-technical users. Academic research has focused on decentralized online social network (DOSN) mainly because it presents more challenges. Some Academic research DOSN prototypes examples are: PeerSoN[Buchegger2009], Vis-a- Vis[Shakimov2011], SafeBook[Cutillo2011], PrPl[Seong2010], SuperNova[Sharma2012], and DECENT [Jahid2012]. Ideally, in decentralized online social networks (DOSNs) users can share various resources and information without the need of a central server. Every user is equally privileged participants and providers of