In case anyone else wants to know the answer, I was able to find it elsewhere. If you run this SQL query on your database:
SELECT * FROM wp_postmeta WHERE (meta_value LIKE '%facebook%') OR (meta_value LIKE '%instagram%')
the search results will contain, in-part, a list of all posts with FB/IG oEmbed content. You may have to change the prefix wp_ to whatever prefix matches your database (I had to). And you also may have to specify your database name in the query. I tested that query and it indeed works as advertised. I’m also told that this SQL query might contain only a list of all posts with FB/IG oEmbed content (but I haven’t tested it):
SELECT * FROM wp_postmeta WHERE (meta_value LIKE '%fb-root%') OR (meta_value LIKE '%instagram-media%')
One last thing is that the SQL query results indicated that my site had one post with FB oEmbed content. But when I opened that post, it didn’t contain any rich oEmbed content (i.e. content described in this article, e.g. nice image, nice icon, nice text formatting, etc) All that exists on the page is a standard link to a Facebook page (screenshot). Because of this, I took no action to fix the issue–based on the description of the issue, this particular content will be unaffected.
See this post on the WordPress forums for more info.