Ask HN: Does the network “crumbling” effect exist?

The network effect is a popular buzzword to describe the “virality” of social network services. As additional users join a service it increases the value for existing users. There are some instances where additional users cause “congestion” and decrease the value for existing users but I am interested if people here know whether a “crumbling” effect exists - If users leave a service, it must decrease the value for others and therefore does the network effect reverse and start crumbling?