I think the best thing is for BB to have a database which has tracking on users when they sign, log off, no longer used, if happened after going into bankruptcy that people departed.... Obviously this is all part of coding so its done automatic. Besides, this is a wise thing to do not just in BB but in any business, and it can be useful for many other things besides checking just why number of users are shrinking.
Important thing to consider are the trends of rise/decline of users over the duration of a BB's season. Such as if at the end or beginning of a season, how significant is the decline rate of users, which would happen because many users may give up on BB if they fail to promote to new division for example.