LinkedIn at least shows up prominently in Google search results, not sure about IG. And Google still has some majored walled gardens of its own like Docs, Gmail, and YouTube.
Google picks up some LinkedIn content, but I would venture it’s a very small slice of what is out there. Just doing a google search for several posts I see on my feed, and very few if any shows up. Same for IG (which makes sense as a lot of profiles are private)