It's already clarified and you posted the answer. Reluctantly changing the definition of megathread to threads with more than 20 pages. The rule is very important IMO for the exact reason you also posted. Most gamblers post their posts in these pools threads and predictions and blah blah. A company doesn't want all their users posting in the same thread, they want forum wide exposure.

This is not limited to the "gambling sections", this is forum wide. only 5 posts in the megathreads count.=
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That's true a company always wants to get exposure in the whole forum rather than getting exposure on the threads which are very long and have so many pages. Mega-threads are good for discussion purposes, but they won't help a brand to gain much attention, and for that reason, it's always advisable to diversify the posts in different threads so the brand or site may get more exposure from various users. I think you have done something great by increasing the limit of mega-thread pages to 20 as that's a good number for the ones who post in gambling board or gambling discussion board because in those boards a thread gets so many posts in a short duration and that's why 20 pages is sufficient for every user of a campaign to   post their responses in such threads.

I also believe that posting in new and fresh threads helps a brand or site to get more exposure and most of the companies want to hire those applicants in their campaign who try to post in threads with less than 5 pages. I think other than gambling board, there won't be any issue for users to follow such rule because 5 pages are more than enough for a user to share his/her opinion regarding an issue and more than that isn't needed in most threads.