I think that you're spot-on regarding diminishing returns- anyone who is relatively active on these forums likely know at least vaguely most of the other regulars and more or less what comic/novels they're working on and have given them a peek if they're looking for new reading material/if it sounds like something that they would enjoy.
To that end, I would say that it's not super super effective in the long term? Or rather the number of people you will attract falls off after a certain amount of time.
This is why a lot of regulars, at least, mostly "promote" in topics that have fun sharing or game aspects to them. Linking your series is somewhat secondary to having an excuse to share something about your characters/worlds/stories/etc. Or at least that's how I view them 
There is something to be said about catching new people over time though. People do come and go from these forums, so having your links out and about every so often can potentially reach folks like that. But generally... yeah, there are only so many people here and at least some of us don't have as much time as we would like to consume more and more stories
/looks at my 100+ unread update notifications
