過疎

過疎 is the standard word for extreme sparseness, above all the depopulation of rural regions (the Digital Daijisen sense, shown on Weblio, with the antonym 過密). The online use is a figurative transfer of that image to a community with few participants. The page opened for this entry documents only the standard sense; the online sense is not shown there, and no dated first use is documented. The verb 過疎る ("to become deserted") is already a separate entry; this entry is the noun form, often used as a plain assessment such as 過疎ってる or 過疎板.