Facebook is a social media site that is intended to allow users to connect with one another. It has since become the largest social media site in history, with an estimated two billion users.
Relevance to Super Smash Bros.
Since the growth of Facebook, especially 2012, it has become one of the most popular ways for smashers to connect with one another. Facebook's ability to allow users to create groups has allowed for such groups to be devoted to entire regions and for specific games, allowing prospective players who are interested in finding tournaments near them or connecting with other players in their region to learn the game. Facebook's popularity has since eclipsed that of Smashboards, owing to its greater presence in most people's lives, it's more well-known interface and etiquette compared to forums, and its ability to streamline groups that allow players to access regions and games pertinent to their interests.
Applied usage of Facebook has also allowed for a number of projects relevant to Smash to be accessible to a wider number of players. For example, various mods such as Project M and Brawl- have their own pages, allowing users to follow updates on both mods without having to use their official sites or follow their threads on Smashboards. Facebook has also allowed smashers to form groups, where much Smash-related content is posted. A popular example is Melee Hell, which currently holds over 41,000 members and includes many top players such as Mew2King and Hax.
Use by Nintendo
Nintendo runs an official Facebook page, as to promote its various games and products. In addition, the series has its own page, covering the fighters and stages of the series as well as promotions for upcoming games.
From SmashWiki, the Super Smash Bros. wiki