From a user perspective, why does marketshare matter?
It mostly doesn't.
Obviously, size of network effect matters, but that's not the same as marketshare.
Even considering size of network effect, what's more important is *who* is using the network.
What matters most is whether a tool allows me to talk to the people I'm wanting to reach.
If a social media service only has 10 users but each of those 10 users affects my life in monumental ways, I will use it regardless of marketshare.