There are many reasons for this. Some of the main ones are:
1. Facebook. As people already mentioned above, Facebook has killed/undermined many a forum.
2. The S-chassis just keeps growing older and less desirable. Not much anyone can do about this.
3. The drift boom and the JDM/tuner trends have leveled off. The 240SX just happened to be there as the right car at the right time in the previous decade.
4. The percentage of young people in first world countries getting their driver license as they reach legal driving age is at an all-time low. People only keep losing interest in driving and cars, even more so in modifying them. The automakers are partly at fault for failing to get people hooked.
5. Regarding the S2000 comments, a lot of cars that seemed unattainable to many 240SX owners ten years ago are now cheaper. Plus the 240SX crowd has grown up and people can afford the vehicles they lusted after in their youth. Thus also any newcomer who would have settled for a 240SX in the past can now probably find an FD RX-7, 350Z, or S2K for almost the same prices as what S-chassis sellers are asking at the moment. Though even if traffic to those forums may be on a momentary upswing, it will eventually start declining as well.
In the end, while the 240SX community will never enjoy the same popularity it did at its peak in the mid to late 2000s, it will at least hopefully go back to quality over quantity with members who are in it for the long haul, instead of being infested with fly-by-night missile cars.
And for anyone saying new members are intimidated, you were obviously not here back in 2005-2008.
