Health apps fail to hook users because of reactance - a a motivational reaction to offers, persons, or rules that threaten or eliminate specific behavioral freedoms. Occurs when a person feels that someone or something is taking away his or her choices or limiting the range of alternatives. The effect is an opposition to the offer. Fitocracy is one of the few fitness apps that instead of nagging users with reminders of being unfit, matches them up with others who enjoy the same sport activity.
There are different reasons for (and approaches to) tracking, so I wouldn’t expect one service to “dominate”.
That said, there are a lot of “me too” products on the market that don’t have any unique benefits, so there will be some “consolidation” as the market matures…