"Against stupidity the gods themselves contend in vain."
My theory on the matter is that the "rank" you have is determined by an element or combination of elements from halo's various ranking systems. Please note that all the following are completely unrelated theories on the B.net ranking system and may be mixed and matched together to form whatever B.net rank system you find ideal.
The "Arena" Element:
In Halo Reach's arena mode, you are rated based on how you stack up compared to others in the same game. For instance, in Reach, the guy with a K/D/A (kill/death/assist) ratio of, say, 1/1/12 could end up with the same rating as (possibly greater than) the guy with a K/D/A of, say, 12/1/1. This might be applied to the forums in the form of "T/B/P," or Threads made/Bans/Posts, if that makes any sense. This, like the K/D/A ratio, would make a daily rating for you, and after X amount of days, you can be rated.
The key element here is that in Arena, it isn't "If you rating is above X, you are Onyx; is it's below X, you get gold." The system there is if "If your are in the X% of players, you're Onyx, below X% is gold." Given that there are only Y amount of Mythic ranks available, it stands to reason that it is a case of percentage, not number, just like arena.
The "Trueskill" Element:
In the older halo games, truskill played the key role in rank, and that trueskill was determined by how many games you won, how many you lost, or how many were tied (for the most part). This number, once determined, was incredibly difficult to raise or lower, and made it nearly certain that 50's were either skilled or were Boosted accounts.
The elements I believe could be in play here would have to be both the "Skill System" and the "Sticky Rank." The skill system would determine how many hot topics (wins), locked topics (losses), and average topics (ties). A tie would have no effect on your rank, a loss would lower it, and a win would raise it. This rank, once determined, might be difficult to change.
The Reach Progression System:
This is a relatively simple element to understand. The longer you're online, and the more you post (or maybe how long your posts can get), the more "XP" is added to your rank progression.
The Halo 3 Playlist Progression System:
Again, relatively simple system. The halo 3 "playlist rank" system is more or less the Reach system, but applied to individual playlist ranks. This may be compared to the many forums on B.net. It may be possible that you "rank up" for getting to a certain rank on all the forums.
Non-halo related theories:
The "Keyword" Element:
The idea here is that all your posts on B.net are autonomously monitored (they* are ALWAYS watching you. You cannot run. You cannot hide), and that certain words, phrases, or terms may discredit or add credibility to your overall post record. Terms such as "inb4," "over 9000," or "FUUUU-," would all be "discreditors." (NOTE: those 3 are just a few that I got off the top of my head. There may be thousands more).
The "Forum Specific" Element:
I find it a pretty simple belief that the forum you post on can and does effect how much "experience" you receive per post.
For instance, a hierarchy would go, "Community Forum>Flood Forum>Halo 3 Forum."
This is because the Community Forum is literally the place where people go to ask the Ninjas stuff about the rules of other forums. Therefore, it is the place where Ninjas go to answer, so there are far more Ninjas here than in most (if not all) other forums. Subsequently, trolls, flamers, and other "illegal forumgoers" are found and punished faster than other forums. The final result is that the only way to get away with posting on the Community is to follow rules for fear of a swift and immediate ban. Honestly, there is nearly never a time in which there's not a thread on the first page with a reply from a Master Ninja.
The Flood, I would say, is an average forum (thus, the middle spot in the hierarchy). It has medium amounts of posts, and you're average dose of Ninjas, usually. Thus, "illegals" run medium risk and receive medium "XP."
The Halo 3 forum, these days, is all but abandoned, so there are nearly no threads made, so there is nearly no need for ninjas, so troll run nearly no risk. Likewise, they receive the minimum possible points per post.
TL;DR: There are many possible elements for Bnet ranks. Many might be related to halo ranking systems. there are many other elements possibly involved, as well.
*By they, I mean the Bungie Webteam.