On any game, the people who post on the forums will be a very small very vocal subsection of the player base. I'm willing to bet that between 50%-75% of all STO players have never once posted.
On this game, this effect is exacerbated by PvP being a small quiet backwater of a generally PvE game. Never assume a small number of posters means a small number of players, or even worse that a small number of posters means the game isn't fun.
And yes forums pvp or otherwise usually do have the same groups of players, most never deign to grace our inflamed threads, apart from trolls, attention seekers and the hopeful minority of those that actually care.
PVP in-game has nothing to do with how many users are on the forum. Have you tried the OPVP channel? It is pretty active during peak hours. Queues have been popping pretty quickly lately, too, and will only get better once the freeloaders make it to VA.