Speaking as a 33 year old with a freshly 18 year old sibling I'd say TikTok fills a similar niche to YouTube. You aren't really chatting on TikTok even if some people might post. What you're most likely doing is finding videos which you then share via another medium like Discord. Discord+YouTube+TikTok is a huge thing with younger netizens (and of course gamers in the case of Discord). YouTube is still popular but TikTok is less censored (this is probably the wrong word since YouTube has edgy content, but it seems to recommend more sanitized content than TikTok which most heavy users I've known agree has a much better/unrestrained recommendation algorithm).

So saying people use TikTok directly to keep up with their friends might not be the full picture. But I'd still say for many people TikTok plays a role in keeping up with your friends in that trading TikTok finds is a popular activity for many friends.

Exactly. I don't care what my friends do on Netflix or Youtube or TikTok or even Instagram. My life and those of my friends don't much intersect with the digital video entertainment we consume. Facebook and text messages are where I interact with friends. On neither do I see non-friend stuff nor ads (I use FB on desktop with filters)