Early on in the series Lost (good read) I noticed that in all the flash scenes other survivors on the island were noticed. It might be Hurley in a television set in the background or a direct interaction between two characters at the airport (like Hurley and the science teacher).
I began to think that they tired of continuing such a game as it became harder to link people in the flashbacks to people on the island. Last night with the appearance of Desmond as the only person in the stadium I knew my theory and fallen apart. Perhaps I was wrong!