I think that common interest is like the edge of the blade.
For example if your social media presence is like a knife (not a good analogy I know)then 'common interest' would be the edge of the blade.
Why? Simply because if a fish needs water to survive then your social media presence needs like minded people to grow. I mean without shared interest what chance do we have of a fruitful discussion?

PS: I am not all that much into social media and it is just my observation that I present.