Asymmetry of connections is a better representation of human relationships than symmetry. We are not as interested in everyone who is interested in us.
Twitter does this best by making the connection on the network unidirectional. I can follow someone without them having to follow me. If I am interested in you it doesn’t mean you are interested in me.
Facebook realized this and solved it by introducing the news feed algorithm. They later added the ability to follow someone without having to friend them. This was Facebook way of admitting asymmetry of relationships and trying to reflect that on the product.
Not all asymmetry is represented equally. Twitter uses a discrete binary model of representing asymmetry. I am either following you, you follow me, we both follow each other, or none of us follow the other.
Facebook’s representation is continuous, and it keeps oscillating between both ends of the spectrum. The algorithm keeps calculating how much I am interested in you and how much you are interested in me. Those separate scores determine how much the algorithm shows me from what you do and how much you see from me.
In hindsight this is what makes Facebook more successful, it is having this more accurate representation of asymmetry of relationships reflected in their product.