I see Groupon's rise and fall as similar to MySpace. traffic for rival deal site Living Social rose 27% Maybe Living Social is the Facebook in this comparison.
For me and my social circle, Living Social is something we consistently use. Most people I know have actually unsubscribed from Groupons mailing list.
Anecdotal for sure, but I think there might be at least a kernel of trend in there somewhere:)
Social sizzle like what LivingSocial adds I think is nice, but forgettable (no one I know has gotten/used the "free deal" group buy).