I only received my AncestryDNA results a few weeks ago, and so I am still learning about its functionality.
One of the options that I find the most useful is its Shared Ancestor Hints because these seem to be the quickest and easiest ways to uncover living (or at worst recently deceased) cousins who may be (or have been) co-custodians of the family history of our shared ancestors.
I can understand easily how the number of Shared Ancestor Hints might creep up as more people test with AncestryDNA and more of those tested link their DNA results to their Ancestry online trees.
However, I have noticed that my number of Shared Ancestor Hints has dropped from a high of 24 to its current 20.
I can understand losing one or two of those with shared ancestry, by them belatedly choosing to unlink their DNA and tree, or turning their tree to private, but are there other possible explanations as to why about 17% of my Shared Ancestor Hints may have "evaporated" during the course of a few weeks?
Two days after posting the above my number of Shared Ancestor Hints has jumped to 29!
Since reaching 29 over the next 6-7 months my number of Shared Ancestor Hints has crept slowly up to 47 with no more unexpected dips and jumps, so I think Ancestry must have been tweaking their algorithms earlier.