That isn't even the issue.
The issue is if people want to stop being on a website then let them.
It's not selfish for them to leave (even without a warning or a goodbye).
It's selfish for you to hold them back in any way because you just want your needs filled, not theirs.
This is what the Internet has taught us, only think for ourselves.
And if need be, work in a collective group to achieve universal goals (which is still selfish for each individual replies on the other to help them fulfill their part).
Also, it's the whole Bandwagon affect.
They want to makes threads so everyone can see, when they could have just sent a PM.