Phone lines are as neutral as reddit. They just cost a lot more to install. So if that's the critical difference, then the above remark about web 2.0 reduces to: it's cheaper now to start a startup.
"A naso", I would say that the underlying economics do differ somehow.
They're obviously different for 'end user' applications, even though network externalities touch on those through file formats.
For phone lines, the network effects are obvious: the value is in 1) connections to people you want to contact frequently, and 2) the potential to contact a wide range of people. No money is made from the actual content, though.
With the "social sites", though, they're aggregating massive amounts of user produced content (most of it pretty insignificant on its own), and making money from that.
At least that's my quick look at it.
I guess people get enough value out of it to not worry too much, but there is something to the original quote.