Retention is about how often your users return to use your product, and this can make a big difference in network effects. If you have low retention, then it will be very difficult to capture any sort of network effects even as your userbase grows, because the overall usage of your product won't increase and you'll lose your defensive position.

In the early days, Facebook had sky-high retention rates. People who signed up kept on using it, so there was mounting value from the network because if you joined later than your friends, chances are high that they'd still be using it and you'd get value from joining. If retention was low, then if you joined too late you'd have missed the trend and you'd be much less likely to stick around yourself.

Networks without effective retention have weak network effects, and weak network effects lead to low retention. Remember that network effects don't come from the size of the network, but from overall usage.That's why growing a network's size isn't enough, it's also important to focus on increasing usage.