Joshua Porter, who coined the term 'the Lesson', has gone to the trouble of further articulating the principle

What Joshua did by inventing the term is to encapsulate the root cause of tagging's success and and other social network effects.

I love it. Since his original post introducing the term I've been quoting it on a regular basis both in real-world conversation and on my blog.

In honour of the meme, here are my blog manifestations:

The Lesson:
"Personal value>> Network effects>> Personal value"
Tagging behind the firewall:
"What's in it for the taggers? (the Lesson)"
Corpus and the Anatomy of Words, Tags and Clusters
"a) tagging's primary use is for personal gain (bookmarking) with network benefits as a secondary (but very powerful) effect (the lesson)"