Facebook - 1.7 billion users. The largest social networking site in the world, by a considerable margin. In addition to personal accounts, you can set up public "pages" for an author persona or activity, as well as create public or private groups for a specific purpose. The upsides to Facebook are that there is a huge audience, the ability to separate your private and public personas, and a lot of versatility (no character limits on posts, the ability to integrate video, photos, and other media, etc.). The downsides to Facebook are that it can be extremely difficult to stand out from the pack, and Facebook's algorithms often make it difficult to seriously promote or advertise your presence without spending a lot of money.

Tumblr - 555 million users. A multimedia blogging network, Tumblr allows for the posting and reposting of varied content including text, photo, video, quotes, links, etc. This platform has gained a lot of popularity among younger audiences for the diversity of content and different ways the platform can be exploited.

Instagram - 500 million users. Primarily a photo and video sharing platform, with the option of entering a caption for the photo. This is a great option for people who aren't really enamored with the idea of expressing themselves publicly in writing. That might sound strange for a writer to not want to write to express themselves, but if you're writing all day it can be a welcome and refreshing change to express yourself in a different way when it comes to your social media. Not to mention that fact that it's relatively quick and easy to snap photos of your daily life and post them while you're running around.

Twitter - 313 million users. A microblogging platform with a 140-character count limit for posts. Photos and videos can also be uploaded. Twitter is excellent for short communications when you just want quick comments or interactions. The downside of Twitter is that it can be difficult to express complex thoughts and have extended conversations in this abbreviated medium.

Snapchat - 200 million users. An image and multimedia messaging platform that gained popularity for it's timed, self-deleting content. Unlike other platforms where your content stays up forever (or at least until you manually delete it), snapchat content is short-lived thanks to it's automatic expiration. A lot of users like Snapchat specifically for that laid-back, natural form of communication where they don't have to worry about a silly comment being dredged up by a critic years from now... but the ephemeral nature of the platform also makes it difficult to communicate with a casual audience if you're not regularly posting and they're not regularly checking in.

LinkedIn - 106 million users. A business and employment-based platform where you can post your resume, blog posts, and connect with other professionals in or outside your industry. This platform can be great for professional connections and there are certainly a lot of features to use, but it has considerably smaller audience base than some of the others, and a lot of people use LinkedIn exclusively for business or employment matter, so the opportunities for social interaction with your public audience is even smaller.

Pinterest - 100 million users. The platform's CEO categorizes this service more as a "catalog of ideas" rather than a "social network," but the general idea is that users can create themed "boards" to which they can post various types of content (images, video, links, text, etc.) that they feel fit the content of their boards. Other users can then subscribe to your boards and see the kinds of content you're posting about. The upside to Pinterest is that is can be a very creative, interesting place full of diverse content... the downside is that there's not quite as much of a focus on direct interaction as the other social networks. In addition, the demographics are heavily skewed; a 2012 study found that as many as 83% of Pinterest users were female.

Blogs. While this isn't a specific social media platform per se, blogs are still immensely popular ways for people to communicate online. You can pay to host a blog on your own personal domain, or host one through a free blogging service like Wordpress or Blogger. The upside to blogs is that you can post pretty much whatever content you want and aren't constrained by the limitations of a specific social media platform. The downside is that there's much less of a communal aspect to the blogosphere which means you're probably going to have to find a way to build your own audience. But, with an estimated 152 million blogs online as of 2013, there's no shortage of places on the web where you can connect with someone who's writing about a topic that interests you!