Through social network sites (SNS) are between the most popular sites in the Web, there is not a formal study on their functional features. This paper introduces a comprehensive list of them. Then, it shows how these features are supported by top 16 social network platforms. Results show some universal features, such as comments support, public sharing of contents, system notifications and profile pages with avatars. A strong tendency in using external services for authentication and contact recognition has been found, which is quite significant in top SNS. Most popular content types include text, pictures and video. The home page is the site for publishing content and following activities, whilst profile pages mainly include owner's contacts and content lists.