I was curious about this as well. I believe Google started indexing tweets in 2014. Twitter gave Google a slice of the data well before then.
If this is correct, I'm curious if those unindexed URLs were Twitter.com URLs (tweets) or 3rd party URLs, meaning those pages were shared on Twitter before Google indexed them.
I figured a huge driver of this were pic.twitter links, since there would be no way for Google's crawler to see all of those URLs without access to the firehose.
If this is correct, I'm curious if those unindexed URLs were Twitter.com URLs (tweets) or 3rd party URLs, meaning those pages were shared on Twitter before Google indexed them.
Edit: grammar