Good question. The choice of Social Media sites should be function of your marketing objectives and target audience. You should segment your target audience based on various parameters such as age, gender, profile, interest & location and based on the segmentation you should identify most relevant channels.
Interestingly, Facebook & LinkedIn help you validate if these are right channel for your business through their ads interface; without even running an ad. For e.g., if you want to find out how many male, aged between 25-35, who are college graduates, from navi mumbai are on Facebook, you can find the number. Similarly, you can use LinkedIn Ads interface.
For search, you can use Google Keywords Planner to find out how many people are searching for a keyword in given location every month.
For other channels, I suggest that you do following:
Look for your competition; if they are active then that channel is likely to be relevant
On content marketing channels such as YouTube, SlideShare: if you find a lot of content being consumed related to your industry, it means that channel is relevant for your business.
Hope this helps? Let me know if you've any further query.
Btw, I suggest that you update your profile with your Picture and your complete name.