I have sent similarly worded notices to both flickr and pbase, although I did not specifically cited the DMCA provisions mentioned in the article. In every case the infringing accounts were terminated within a few days.
OTOH, several local businesses have "borrowed" my images without asking or credit, and certainly without payment. Email and phone contacts have generally been ignored by various creative means, so I think I'll give the ISP approach a try. Thanks for that info!