I think there’s another element that increases churn as well. Both musical styles and individual social networks end up having strong ties to particular generations in a manner that can actually alienate younger audiences.
This is the “not your father’s Oldsmobile” effect: if a social network is dominated by an older crowd (or one that’s been there for a number of years) it becomes less attractive to a rising younger generation looking to carve out their own identity. The same thing goes for musical styles. The genres that were generation-defining when I was in middle school (grunge and classic hip-hop) are now played on retro radio stations and have been completely stripped of the parent-irritating novelty-rewarding qualities that made them appealing to kids in the first place.
The new web-based cultural communication channels, with their massive throughput, have been speeding up this cycle by shortening the time it takes for youth trends to spread widely and hence become watered down. Simultaneously, they also greatly reduce the switching costs for both media (where discovery is the main cost) and social networks (where efforts like google’s open social and the growing standardization of feed syndication are reducing lock-in). Both of these factors will increasingly penalize the labels for the slow response times they demonstrate because of partnership contracts and a general hostility/ignorance towards the web. In fact, I often wonder if this process has not already achieved escape velocity and permanently broken out of the orbit of the major labels and the other legacy distribution businesses.