It is known that the network externality plays an important role in the IT services. In terms of the effect of network externality, it is sometimes said that the firm with the largest share in a saturated market is more stable than other firms with less shares. In the case of the cellular phone market and Social Networking Service in Japan, however, the share of top-share service providing firm does not seem so much stable. This might seem contradictory to the previous researches. In this paper, we study the effect of the new attractive technology to the market share based on an agent model. It is shown that the firm with less share is not so much in disadvantageous position compared to the firm with the largest share, and that the real share change can be explained within the model.