Why social media? Because that's where people are increasingly getting their information.
Where should you be on social media? You don't need to be on all platforms, but you should be where your constituents are. For most, this will be Facebook, and, increasingly, YouTube, but Twitter and Instagram are useful at different times in different ways. (It's useful to see the chart below for the different demographic social preferences.) Anecdotally, anyway, I would say that Twitter is a good platform for reaching journalists, for example (check out the #mapoli hashtag), and Instagram is good for rallying good feelings around an event or cause.
Congresswoman-elect Alexandria Ocasio-Cortez (NY-14), for example, has made great use of Instagram in the time between her election and actually taking office. With 1.1 million followers, she is reaching a national and global audience with her message. People even watch her cooking dinner. Don't laugh; more than 250,000 people watched Rep. Beto O'Rourke make dinner on Facebook Live.
How should you think about social media? As a core component of your communication strategy, rather than an add-on. And you should be thinking mobile.
It's also important to keep track of tech companies--and the public's relationship with them--is changing.
The platforms themselves are also always changing, so it's important to keep up with the social media industry, with sites like Social Media Today, Digiday, and social media demographics with sites like the Pew Research Center. Social media management takes time and commitment. You're better off with one channel that is well-maintained than several that are not.
Here are some basic resources and information about the demographics of social media.
For policy perspectives, The Pew Research Center is an excellent source to keep up with the intersection of technology, media, social media and the workplace.
WHO USES WHAT?
Different social platforms have different demographics. This chart comes from the Pew Research Center. Here's a link to the full data.