My friend in Chicago has responded so here's his take on positive externalities (reproduced as-is)...

"here's a proper defn: When you produce or consume a commodity or service within your private property rights that bestows a benefit on a third party not directly involved in the market transaction.

in english: u do smth for ur own benefit but others indirectly benefit. i like the fax eg: as more ppl buy fax machines, existing fax users gain 'cos they can connect to more ppl. that gain by existing users is terms positive externalities; negative externalities would be the opposite.

thus, as more ppl blog, existing bloggers gain as they can connect to more ppl. positive externalities, dont u love econs? :)"