A washingtonpost.com article from: millerk at nytimes.com

[Mark A Mandel]

"Flash crowd". The term was coined AFAIK by the sf author Larry Niven in
a short story, whose title I don't remember but other fen* certainly
will. It's set in a near future (a few centuries or less) in which
cheap, reliable teleportation has supplanted all other means of
transportation.

Example: a tv (or equivalent) newscaster says "I'm at Xxx Beach and the
sunset is simply stunning!" People step into their home teleport booths
and transport themselves to the public booths nearest the beach. In
minutes there's a crowd of thousands or myriads.

This is today's version. We now have flash crowds.

* a plural of "fan"

-- Mark A. Mandel