While Google + was not highly adopted at first, Google made sure to make it a force to be reckoned with. Once people started to realize the effects of plus on search engine optimization profiles started flying up. I'll admit I wasn't one of the early adopters of a plus profile, but now that I have one I see the benefits. People who use Google + seem to be more interactive with the content you promote there. Not only will you get a boost in search results you will have a great social profile for finding the information you like, that pertains to you. Using plus is great and don't forget to add your authorship markup, you can find how to do that here.
Ah yes Facebook, probably Google's main competitor in the internet world for the foreseeable future. Facebook is a great place to build a unique band of followers to your brand. Not only can you do this, but by increasing your followers you will increase your results in search. Now Google says "these social signals help, but not that much". Well I say they do, this is due to my real world experiences and I see my content get a boost in search from increased sharing on Facebook.
Twitter in all its might does do good things for websites and allows fast sharing to a large community if you build it. However with tiny links and the results of no follow the boost you get in search is minimal. However Twitter is a great source of traffic and back links to your page.
Pinterest is an emerging force in social media and if you aren't already taking advantage of the free images you can get for your website, you should be and if you have good images you should be sharing them. I have gotten lots of traffic from Pinterest, the thing is other webmasters and like minded people search for your photos, if the photo is good they like to see if there is more, or what the rest of your website has to offer.
Other Social Networks
I will never say "don't use a social network", but you should only have as many accounts as you can stay active with. Activity is very important on your website and on your social accounts. So yes use them all, try them out and see what works for you. If you find the holy grail for your business great, if you don't well you will and it may just take some time. In conclusion, be social stay social and share!