Social media lunching their own cryptocurrency would be a plus to the cryptocurrency industry as it would make cryptocurrency more popular as most persons would be forced to learn how it works Inorder to use premium functions on the said social media. The coin market cap would also experience flow of new funds from these social media and since bitcoin is the most accepted coin, it would likely lead to an increase in the value of Bitcoin.
In theory such a thing could bring new people to become part of this market, but I doubt it would happen that easily, right now the first coin that people adopt most of the time is bitcoin, if a social media website which was extremely popular released their own coin then it is to be expected for this behavior to switch.
However once people could use that currency to buy and sell on that social media platform there will be little incentive to adopt bitcoin as their needs will be already met, so on average the adoption of bitcoin should slow down instead of accelerating on this scenario.