We went through this last fall with out cat. Took cat to Petco for a dip, flea powder all over the house, on the couch, beds, etc., vacuumed it up. This didn't get rid of them.
Took the cat for a 2nd dip at the vet, called our exterminator. He came in with something, walked through the house spraying everything, came back a week later and hit it again. Did around the house as well, no more fleas.
So far so good, we get a 5-month supply of the flea treatment and apply it each month, works very well.