If they seem to be part of the production, they’ll show off to their friends.
Include the following in your newsletter, something like this: “With a few clicks and in one minute, you can have your very own version of this newsletter. You can pick and choose what to include in it from a variety of articles, videos, quotes, etc. Then you can share your very own version with anyone you think might like it. Click here to get your own version where YOU are the acknowledged editor.” When they click they see a web page that shows the same newsletter with options to change a few parts of it, add other content that you already have for them to include, and even let them write a paragraph or two of their own. Then they can either type in email addresses of who they want to send it to (or use their contact list), or send it to themselves to be later forwarded to others. The newsletter is showing (and is sent with) their name prominently shown as the editor, just like the original newsletter they received had someone’s name showing as the editor.
To find out when and how it was created and gained momentum, and see from multiple points of view its speed (chronological, geographical, generational, etc.)
Analyze online sources of data to find the first occurrence of a word or phrase and plot the spread and usage.