I've never NOT had success in battling these things with Mosquito Bits... very simple to use.. soak the granules in water for 30 minutes, strain them out and use the water to water your plant. Use it only once a week for a maximum of 3 weeks (for the heaviest infestation)... no more fungus gnats... The damned things live in the soil itself so just setting out fly paper won't do a thing.. Mosquito Bits will kill adults, eggs, larvae... and it sure sounds like you've got a massive infestation which will eventually lead to them killing your plants if you don't do something SOON... they feed on decomposing materials in the soil but once that stuff runs out, they start attacking the roots themselves... by adding the compost, you gave them more to eat so they'll stay away from the roots for now but that certainly won't do anything to kill them - it only feeds them!
Mosquito Bits. Now.