You've just provided a helpful example showing that in systems with network effects, the effects are not always positive. Fax machines—to pick a now-archaic example—are more useful as more people own them, but the fax machine's ubiquity also made it susceptible to junk fax. Email is the same way.
In a system with multiple network effects, users' experience of the marginal added value—or pain—of additional members on various effects may create limitations to the ultimate maximum size of a network, or perhaps even doom a network to oblivion as the pain floods out the benefits.
People's perceptions of network effects—positive or negative, and how much so?—can also change over time. E.g. we become desensitized to pleasure and pain.