Evaluating which social platforms to engage with your customers will entirely depend on:
Whether your market / customer base already use that social platform.
Getting traction on a social platform that has a very low amount of users in your industry is next to impossible.
You will notice generally that
1. Facebook is for: entertainment / videos / funny stuff.
2. Google plus is more informational, conversational. This lends itself to more educated topical conversations.
3. Twitter is about news: breaking updates, politics, and trends in breakout.
4. Instagram is purely visual: great for fitness, cars, anything highly aesthetic.
5. Pinterest is about women: great traction for personal care, cosmetics, anything women orientated
Go find where your industry hanging out. That is the best place to engage socially.