Twitter
Twitter users say it is a great way to find collaborators, promote your own research, and keep up with what's happening in your field. Check out these links for information on how to do that:
10 ways researchers could use Twitter by Salma Patel
An introduction to Twitter: becoming a networked researcher 2015 by Ned Potter
More information on the use of social media: Social Media for Academics, an e-book by Diane Rasmussen Neal
Example of a Twitter feed: Check out the feed of cognitive scientist Steven Pinker.
Blogging
If you enjoy writing, and are willing to make blog posts on a regular basis, you can set up a blog or get together with others in your field to do so. While it's not clear whether blogging or tweeting your articles will lead to more citations, some researchers have reported that doing so increases downloads. Several researchers have written about why they blog; see for example, I am a blogging researcher: motivations for blogging in a scholarly context and Why do academics blog?
Others have written about the potential downsides of blogging.
Other resources: How to create a successful science blog by Kelly Oakes
Blogging for Researchers by Helen Dixon
Examples of blogs by academics: Check out The Best Academic Blogs, an article from The Atlantic. Note that many others are listed in the comments on this article.