(Disclaimer: I work for a real-estate tech company other than Redfin; my opinions are my own)
Brokerages also have network effects; they just end up being nullified by the MLS. If some real estate website somehow cornered the market on listings by dominating the seller side, they could have a significant network effect by either not publishing to the MLS or doing so after a delay.
Which is what broker groups on strong redfin areas are basically doing through collusion. They run "silent markets" by delaying listing new houses on the MLS, and giving traditional brokerages not working with redfin early notification of listings.
Brokerages also have network effects; they just end up being nullified by the MLS. If some real estate website somehow cornered the market on listings by dominating the seller side, they could have a significant network effect by either not publishing to the MLS or doing so after a delay.