Someone is always going to be the biggest, you can't just label something popular as a monopoly and then say "the smaller competitors don't count", of course they count, the entire problem with a monopoly is that competitors can't survive because the monopoly has exclusive control of the market, but this isn't the case with websties, if my site has 10 million users it isn't in any way impacted by the fact that another site has 100m users.