It’s not the crawling that is the issue. Facebook for example shows up just fine on search results, you just need an account to view the content on FB if it isn’t public. User privacy settings also shield content. Google has actively and willingly destroyed its utility as a search engine by placing revenue sites/preferred results ahead of actual search results. What they want you to $ee comes first, and if social media like facebook isn’t profitable to google, you don’t get to see it. I did a quick search for “facebook car groups” and got a page full of them, so you absolutely can get those walled gardens in results.