Snapchat has nothing to do with a general 'growth slowdown'. People have left Snapchat for TikTok[1]. Social media users, especially youth, are fickle and the industry relies heavily on network effects. Once you're no longer perceived as cool and/or lose enough users that the network effect isn't as powerful, it tends to snowball, and is hard to get that back.
"But there could be a bigger reason the company[Snapchat] has seen revenue deflate. Its reputation as the social media platform for young people has faded—particularly in the eyes of advertisers."
Not denying that growth slowdown is happening to some extent. But Snapchat has problems beyond that, and isn't a great example for it.
[1] https://www.bloomberg.com/news/newsletters/2022-09-01/snap-i...
As for Roku, I'm not as well-versed there, but I do see that in October 2021 they announced that they're losing 85 channels, including the behemoth YouTube, so that may have contributed significantly to its decline this past year.
Not to mention lots of TVs now tend to have the same type of channels as Roku built-in, rendering it a bit unnecessary. Mine has everything I care about for streaming, for example. Netflix, HBO Max, Hulu, Disney+, etc.
[2] https://bestlifeonline.com/news-roku-losing-channels-youtube...