My sister and I will frequently have long conversation threads on livejournal, holding two conversations simultaneously; one in the "message", and another in the subject.

Occasionally the conversation in "message" will reach a natural end, while the "subject" conversation will keep going, leaving awkward half-hearted messages in the comments.

The opposite is never a problem; subjects are completely optional, so low-effort subjects are fine. They're a step up from "no" subjects, even.