There has really been an explosion of ATSs of late, and I think the reason is new best execution policies and demand for lower trading and implementation costs. Most of it is focused around large block orders, dark pools and institutional liquidity, etc.
I don't know how profitable they all are, at least initially. BATS was recently ranked #1 in speed and price improvement, but I read that they lost $6 million in January from price cuts to compete with Nasdaq.
There's a lot of fragmentation happening, which only complicates the process of finding liquidity...
