I do not think Instagram has reached its peak as a social media. Although Instagram has great features, it still lacks in some aspect, and if it does not fill that void, another social media platform will take its place and give the public what they want.
This will, in turn, cause a decrease in the use of the app for a company to reach its peak, it means the company has filled all the void and are way ahead of their competitors, this includes creating new features that will make the usage of the app convenient, and worthwhile. However, Instagram is lacking in a lot of features that its competitors are having. For example, the Chinese website allows you to exchange money and still chat with other people and share pictures.
It is hard to tell when a company or product has peaked and then will go downhill or plateau. In the case of social media sites, usually the only way to determine whether they will continue to succeed is to look at the competitors. If the other social media sites like Mobli, WeChat and WhatsApp start to become more global especially in the United States, that could impact the success of Instagram. However, at this time, Instagram seems to be increasing in popularity.
I do believe that it will start to go downhill as more and more social media sites are invented and introduced with more functions that the users may want. For instance, WeChat allows photos to be posted, messages can be sent and money can be exchanged.