I'm going to just place one little grain of salt here:

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Take as necessary with the above comment. :)

Xango is by no means on the decline, in fact, things are better than ever!  They have expanded their product line from just the amazing Xango juice to include supplements for the four pillars of health to bring your entire body back in balance so you can truly live a healthy life!  There will always be companies that come and go, but Xango isn't one of them, it'll be around for centuries to come!

They apparently spiked upwards during the holidays.

MLM Popularity Top Gainers this Month

I have not heard much about XanGo lately.

Are they on the decline? Has the health beverage market been oversaturated with newcomers and competition?

According to MLMRankings.com the trend for XanGo seems to be declining since 2008.