Is YouTube declining as a social media platform?

No, YouTube is most definitely not declining as a social media platform.
The reason why most social media platforms decline over time is because the users of those platforms are becoming old and they are losing interest in the platform because there's nothing new on the platform for them.

This is not the same for YouTube, YouTube has new contents every day, the contents on the platform appeals to all age groups. There's something for everyone on YouTube. YouTube also has content that covers all spheres of life ranging from education, government, news, business, religion, culture, economy and many more. The list of things available on YouTube is endless.
YouTube is definitely not yet declining as a social media platform.

It is inevitable that social media websites are going to experience a decline over the years. This is because of the population that is growing old. When people become older, they start to not become active on different social media platforms anymore.

YouTube has a wide demographic though so even if there is some decline, it is not as huge as the decline of Facebook and other social media platforms. It also helps that YouTube always has the ability to offer something new to people mainly because of the new videos that are uploaded by its different users. There may still be more rising stars that will be discovered through this social media platform in the future.