This is almost becoming as common a question as "How do I crouch?" in the last version

Not everyone can keep themselves entertained with a game in early alpha development. Some people check it out and don't find it to their liking, so they leave. All those forum users are from 4-5 years ago to now, and clearly not everyone has stuck around - especially not when PlaneShift was just at the Atomic Blue stage. That took a certain special quality.
Plus some people register on the forums to check out the community and ask questions before downloading - sometimes what they learn makes them dislike the game immediately, such as lack of certain features, some game guidelines, etc. Then some people make multiple forum accounts. The forum is by no means a good representation of player numbers.
*edited to add*
The total amount of registered in-game accounts is well over 150,000. From very old to very new.