Does card-counting work in online Blackjack?
Online casinos use an 'infinite shoe', which would preclude any card counting, as they effectively draw random cards with a fixed probability.  This means it's possible (though very very very unlikely) for all the players at the table to draw the same card (e.g. everyone gets an eight of hearts).

This is done both for ease of the game developer (fixed probabilities are easier to code than simulating multiple decks) and to prevent card counting (which is unenforceable on-line).

So, in short, card-counting is completely ineffective on-line.