The decline of welcome "Check out this link" messages is at least partly because everyone is getting the same links from the same sources at any given moment.

If you found it on a surface-level subreddit, a major Instagram account, a major news site, or a big podcast chances are decent that the content in question has already been seen by a fair number of people, and to them, seeing it again has no additional value.

I see this play out hundreds of times in real life where the conversations start with "Did you see...? and "I was listening to a podcast about..."