First stage is to get really clear on your target market – who do you want to engage with or to buy your product or service. You can do this by developing personas to describe your ideal customers – probably more than one – so you can envisage who you are going to engage with.
Then consider your customer journeys – how will your customers ideally come across your offering? are you filling a need that people will actively search for, in which case how can you pop up directly in their searching process, or are you more of a life enhancing brand where you might need to grab consumers attention in another way.
You can then consider the profiles of the different social platforms and decide which one fits best with your customers and how you need to engage with them. it is often quite a binary decision once you are clear on the customers as the digital platforms have very different users and engagement.
There are some great tools from businesses like Hootsuite that you can use to guide you.