I decided to do something similar with the number of member posts to Wisconsin's listserv (WISBIRDN). The data source I used only goes back to 2010 for complete years (2008 and 2009 were fragmented). Wisconsin Birding (FB), the Facebook group I moderate, was created in June 2011. However, now there are at least a dozen birding resources for Wisconsin on social media. A few are merely redundant groups while others are more specialized to a birding club, natural area, or even particular species. Overall, it must be having some kind of impact on listserv usage.
First, here's the membership growth of Wisconsin Birding (FB):
Now the data for WISBIRDN showing month and year:
And WISBIRDN's future?
Correlation isn't causation, but it appears WISBIRDN began to take a dive around the time Wisconsin Birding (FB) was created (red "X"). Other non-listserv Wisconsin birding forums were created shortly thereafter. I would think eBird alerts factor in as well. A few thousand posts per year is still a lot of activity, but the trend is clear. My guess is the Wisconsin's birding listserv will be around for a while yet, but might go the way of the dodo by 2019 or so. Meanwhile the membership growth rate on Wisconsin Birding (FB) maintains its steady climb.
What will ultimately replace Facebook? It's difficult to even speculate. Did the telephone circle and hotline birders foresee a time when their systems would become obsolete and be replaced by an email network? In a way, the two technologies merged with smart phones.