We consider a dynamic model with two horizontally differentiated platforms. In each period, platforms collect data from users that improves their services in future periods. The incumbent platform starts with a larger initial data base, while the entrant platform has a higher base quality. We find that when the value that users derive from accumulated data is low, the entrant enjoys positive momentum-gradually increasing its market share over time and eventually dominating the market. This occurs even if the entrant has an initial lower market share than the incumbent. When the value of data is high, the opposite occurs and the incumbent strengthens its position. We further find that greater product differentiation benefits the entrant, making it more likely for it to gain momentum and dominate the market. Finally, a policy that mandates reciprocal data sharing between platforms leads to long-run coexistence, with both platforms being active in the long run.