You can find public social media posts using Google and other search engines (e.g. DuckDuckGo, Bing).
Find Facebook posts:
Find posts from Facebook or Twitter:
You can use CrowdTangle to find out how widely shared a link has been on Facebook.
The total likes and shares reported by CrowdTangle include actions taken in private accounts. You'll need a Facebook account to use this Chrome extension.
Here's an example: