Of course this is not an exhaustive list, just the most well known.
Facebook — an online social platform that connects people and organizations with friends and others who work, study, live, and engage around them. People use Facebook to keep up with friends, to share links, to share photos and videos of themselves and their friends, and to learn more about people and organizations.
LinkedIn — a business-oriented online social network that allows users to strengthen and extend one’s existing network of trusted contacts. People use LinkedIn to stay informed about contacts and their industry of interest.
Twitter — a "micro-blogging" and social network that allows its users to send and read messages known as tweets, text-based posts of no more than 140 characters. People use Twitter to share and view links, pictures, videos, opinions, news, professional information, personal status and more.
YouTube — a video sharing website on which users can view, upload, and share videos and comments on other’s videos.
Flickr — a photo sharing website on which users can view, upload, and share photos. Popular place for storing photos that are linked to from blogs and other social media utilities.
Instagram — a photo sharing network that allows users to upload photos taken with a phone or tablet and apply special image filters through the app. The photos can be shared within the Instagram network or throughout other popular social networks.
Pinterest — favored by women, this platform allows users to 'pin" images from any web page to boards on their profiles. Common topics of boards are crafts, recipes, home decor, and fashion.
SnapChat -- A mobile app that allows you to send photos and videos, known as "snaps."
Google+ — an integrated social platform that dovetails with YouTube, Gmail and other Google services. Users can network with each other via Circles and engage in conversations via comments or +1s.
Blog — an online site short for “web log” that allows an individual or group to share a running log of personal insights, writings, events, news, and more.
Foursquare — lets your friends know where you are and for you to know where they are. You can also collect points, prizes and "badges."