There could be two reasons for those SL users' perception.
One, they're used to absolutely massive crowds, so they perceive even what happens at big anniversary parties as "only few people here".
Two, like many other OpenSim newbies/non-regulars, they don't know OSW, so they don't know how and where to look up parties and busy places.
If you teleport and/or walk around randomly, it could happen that you meet nobody.
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