On Earth the seas have numerous filter feeding species, large and small, sustaining themselves on the microscopic nutrients and organisms that drift in the current. The largest animal evolution has produced here feeds on some of the smallest. Tropical coral reefs are the go-to example, but as a strategy there are examples from pole to pole and from the sun-warmed shallows to the abyssal depths.
Imagine a world with terrestrial environments in which numerous organisms – both sessile and motile – have evolved to do the same, living off pollen, spores, seeds, and other biological matter drifting through the air. Without necessarily using webs or slime, just opening its mouth or purpose-evolved appendages and scooping the air an organism can sustain itself for a season and/or supplement its diet. As on Earth there are biomes (perhaps the equivalent of tropical rain forests) where this is omnipresent and others where it may be less prevalent (perhaps the poles), but it is essentially as prevalent as filter-feeding in the ocean.
I expect a world that can support this sort of widespread feeding strategy would need prolonged periods of pollen-, seed-, and spore-production, which suggests minimal seasonal variation through the year and a high degree of competition. It also suggests there aren't enough pollinators to do the job manually, or the dispersal of pollen/spore to the air is done to combat pollinators and predators that consume more than they spread about.
Note: I am making a distinction between filter feeding and predation of flying organisms with webs and such, as that is already a successful strategy.
What conditions – atmospheric composition and density, tilt (as above, I suspect a low-to-no degree of tilt), evolutionary history, etc – would directly contribute to the success of such a global strategy and how?