Three points to consider: First, I am writing this in January of 2015. The social media landscape will likely change dramatically over the course of the next few years, so bear that in mind.
Second, whichever platform(s) you choose, social media is a long-term recurring commitment that requires daily updates. Whether you view it as a time suck or a fun way to keep in touch with your community, social media requires presence.
Third, in the early days of Twitter and Facebook, a lot of emphasis was placed on how many followers your brand could accumulate. Today, the sheer number of followers is hardly relevant. What matters is engaged followers. In fact, with Facebook's new paid-only reach, having a bunch of uninterested or spam followers is actually a huge liability. Focus on growing a community, not just followers.
So, let's take a look at a few specific platforms:
One nice thing about Pinterest is that it isn't just about the present moment. Your pins from two years ago will be made current by a fresh repin, which often leads to more repinning. Posts about homemade Halloween costumes, for example, get bumped back to the top of the feed every fall. Users usually don't send messages, so you won't need to be as ever-present with Pinterests as other platforms require.
Pinterest is pretty flaky, though. I've seen our pins get repinned dozens of times with no sales. People tend to collect pins as an ideas that seldom imply any current action. We do see a lot of web traffic coming from Pinterest, but not a lot of conversions.
Pinterest is also very global and isn't easily limited to one geographic area. This is fine if your brand has a national or global reach, but may not be ideal for a strictly local service business.
Twitter is the most challenging and potentially rewarding of social media platforms. A constant stream of consciousness, Twitter lives in the ever-present now. So, first and foremost, Twitter requires an always-on manager that can respond quickly to incoming tweets.
Twitter culture is rich with in-crowd hashtags, humor, and instant memes. Wading into Twitter can seem challenging, but the key is to remember that it's all just a big conversation. (Take a look at our profile for some practical examples.)
Start by following the feeds of your suppliers, friends, competitors, other local businesses, etc. Make a practice of seeking out and following the Twitter feed of any new person or business you find interesting. Then, pay attention to how people talk about your field of interest, which hashtags are used, and who has the most retweets or the most followers.
Think of Twitter as a way to plant seeds by throwing thousands into the wind. Very few will find fertile ground, but they may grow in places you hadn't thought of. There are hundreds of ways to use it, but at its heart, Twitter is about discovery.
There are a lot of great tools available to build your Twitter presence, including ways to find nearby followers if your business is primarily local. And, although it's possible to pay Twitter to promote your posts, the core of Twitter's functionality is 100% free. They haven't, as yet, abandoned unpaid "organic" reach as Facebook has.
Next in part two: LinkedIn, Tumblr, and Google+.