Here are the ecns I'm aware of. Are there any others? NSDQ ARCA EDGE BATS? NITE? Are these ecns? Is BTRD still around?
NSDQ, ARCA, EDGE, and BATS are all full-fledged exchanges now, as are their smaller units such as BX, PHLX, EDGX, and BATY. Have you been away from the market for a while? Google the exchange names and terms like "market share" and you can read about the status of equity exchanges these days...