I don't feel it like a dead game. If you enter the game, you will find plenty of people to group with.
Next year, the game will become 10 years old. It's near the same amount of time between the release of Diablo 2 LoD and the release of Diablo 3, so we can say the game has aged pretty well.
However, maybe diablofans IS dead, but that doesn't mean the game is. You can see plenty of updated guides at Maxroll, and people talking at Diablo 3 reddit. The players have shifted to those pages, and if you go over there you will notice a much alive community. Sorry, not mean to spam, but I think it is important to remark this, as maybe people is thinking the game is dead just because the original main community is dead (which is sad, I used to look guides over here and I love this page).