The four ATS's I mentioned all became full exchanges. ARCA and INET got bought by existing exchange operators, while BATS and Direct Edge were cleared by the SEC to be exchange operators. These four all exist after a massive wave of consolidation among the ATS's from the past decade.

There are also dark pools, whose innovation is that they don't display quotes. Every large brokerage has a dark pool. I wonder if there will be consolidation there; it's anyone's guess at this point.

As for network effects, Reg NMS requires an exchange to route an order to a rival if the rival has published a better price. So network effects are less important than you might think outside the dark pools. The features that really attract market makers are things like fees and likelihood of getting a fill (ie, priority rules).