So say you have a 2 player game, taking Chess as an example (except it's an original game with no ready-to-go AI available).

Let's say there's also a social-aspect to the meta-game, so let's say it's a Chess game on Facebook where you can challenge your friends.

How important is it to have a single-player mode, knowing that an AI will need to be created (I've done minimax AI for tic tac toe, but nothing too sophisticated)?

Is it important enough that it should be in the initial launch of the game?

Can it wait for a future iteration (knowing that being hosted on the web means the game can be updated at any time)?