A recent episode of the podcast Digital Detectives, featured Nathan Russell, a Canadian lawyer with a focus on legal technology. Part of the podcast concerned the collection of social media evidence. Russell recommended a tool called Web Preserver, available at http://www.webpreserver.com/ . Web Preserver can be installed for free, but you will need to set-up an account which is only free for the first 15 days. It will show up as a small blue square icon on your Chrome browser's toolbar.
The app includes the options to save web pages as PDFs or JPEGs with associated EDRM XML files or Concordance .opt / .dat files. There is a bulk capture feature allowing users to enter long lists of URLs.
You can use Web Preserver to track when data from web pages is collected. The online account records the time at which data is collected, and also generates a SHA256 digital signature to uniquely identify the exact social media posting.