They say that online relationships aren't "real" because people can disappear at any time without notice, never to be seen again, and nobody knows what happened. This is no different from offline relationships. Every time someone graduates from a school, changes jobs, leaves a social club, or moves to a different city, people lose touch then, too. The number of people one stays in touch with when leaving any community, online or offline, is about the same.
They say that leaving an online community is like flipping a switch. Turn off the computer and it ceases to exist. This, also, is no different from offline communities. People can walk away from those, too, and never look back to see how their absence affects the ones they left behind.
They say those things and they are wrong, because we do still exist, and we do still care. Even if the one who leaves never does come back, we still have to wonder why.