Assuming this only counts active forumers, the world's population would be ~200 and mostly males <18 years old, and mostly separate, in some cases on different continents.
Forumers would likely either be alone or in a small group of <5. Most major infrastructure would fail to function, so unless you've got solar panels, kiss electricity goodbye. In rural areas, forumers would likely have to farm for sustenance, or learn if they don't already know. Urban forumers would be limited mostly to what they could scavenge from abandoned stores and homes.
It honestly depends on the abilities of the average forumers to be self-sufficient and be able to secure and maintain food, water, shelter, and heat. Considering most forumers live with their parents and are entirely dependent on them for everything, the human race would likely go extinct. The lowest the human population has ever been bottlenecked down to was ~10000 individuals and the fact the human race recovered from that was the more unlikely scenario. With 200 individuals, even IF the forumers were somehow able to cross oceans and band together, the lack of genetic diversity would be a problem and any sort of natural disaster would be particularly deadly to such a small and concentrated population.
Edit: forgot about infighting. There would probably be major issues over who would lead/handle distribution of resources.
