My question comes from the same source as this post on increasing the paid membership base. The last time I checked, we had over 400K users on here, but it would be interesting to know how many of these are "active" journals. Perhaps a friendly communication campaign re paid membership benefits could be started with the most active non-paid users, for example. We could also draw small samples from that population (n=600, say) and survey them on their LJ usage habits in order to discover their propensity to upgrade their membership.
Addendum: I found the stats page - now I'm looking for a cross-tab of "update frequency" by "account type".