This quote demonstrates the emptiness of the people in Montag's society.
The “people who ate shadows for breakfast and steam for lunch and vapours for supper” are the people who Montag left behind. This quote demonstrates how Montag has begun to feel about his society since his exposure to the books.
After his murder of Beatty, Montag had to go on the run and used the river to escape from the mechanical hound. Since his pursuit was being filmed, the government found a scapegoat and the hound killed him. The man’s face was never seen, so even his closest friends would never know it wasn’t him.
Somewhere the saving and putting away had to begin again and someone had to do the saving and keeping, one way or another, in books, in records, in people's heads... (Part III)
Montag finds the people who save the books and joins them. When they see the city bombed, they realize they have to go back and recreate the society that was lost.