You can find public social media posts using Google and other search engines (e.g. DuckDuckGo, Bing).
Find Facebook posts:
nigeria vaccines site:facebook.com
Find posts from Facebook or Twitter:
Use this tool to search across multiple social media platforms at once. (Just like when you're searching Google, you'll only get public posts in your results.)
Here's an example search:
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: