Many of them are not really people, they are computer bots and spiders and things that are just scanning topics and such.
In addition, usually a forum is set to show a person is here for a particular length of time, regardless of how much time he/she actually is here. If, for example, a member comes to check to see if his thread has been answered, but is only 'here' for a minute or two and leaves, the forum still will list his/her name as still being present for whatever time-frame the forum is set to. Common times are 15, 30 min, and even longer. It makes a forum appear more active than it actually is.
In the same vein, we currently have close to 3,500 members. But that number is not an accurate reflection of active participants. Most of those are people who registered, visit once or twice and moved on years ago. Or former active members who for various reasons just aren't interested in figs anymore. Or people who just never post, but are lurkers.