I utilize most of the major social networks - Facebook, Twitter, Pinterest, Tumbler, and Google+.
I was using Reddit but I do not really understand the purpose of it. My account was flagged as spam so I gave up on Reddit.
I do get a lot of random followers from Twitter and Tumblr that really seem to be people pitching their website and not much more.
They are not really interested in what I have to say.
Facebook, Google+, and Pinterest seem to be the most
real. I get more genuine human beings following me from these.
As for their usefulness? I am not quite sure. I have only ventured into the social networking realm withing the last six months or so.
I have seen a larger overall increase in my app downloads and revenue but I can not pinpoint it to either my increase social network presence,
revamping my website, or increase in the use of app stores other than Google Play.
Facebook charges a pretty penny to advertise. Out of curiosity I paid around $50 a few times for a short promotion on of my pages and got about 45 like in a week.
It is hard to tell what that equates to in actual downloads however.
I have paid several times with AdMob.
I do a daily budget of $3 and .03 cents per click. With my Match block app I received about 200 downloads in a week. Usually Match Block get 2 -5 max download per day.
I suppose to get maximum exposure you must utilize as many app stores as possible, take advantage of social networking, and create a custom website for your brand.
If you have the funds then a little advertising never hurts.
Just my experiences...