I login to 23andMe, AncestryDNA, and FTDNA every few days to see if I have any new half-siblings (none so far). This weekend I noticed that AncestryDNA has added an option to see "shared matches" among DNA relatives. It only seems to show matches that they deem "4th - 6th cousins" or closer (which can include 3rd cousins several times removed and double 6th cousins and then some). If you click on what they deem distant cousins or "5th - 8th cousins" it will show you nothing, regardless of whether or not you share that relative. It also won't list the more distant cousins in your "shared matches" with other people. I assume Ancestry leaves them out because accuracy plummets the more distant a relative is, and after you reach the vicinity of 6th cousin or so, you really can't use DNA to tell a distant cousin from another random "unrelated" human being.
AncestryDNA also hasn't added any quantitative data, so you still can't see on which chromosomes you match like you can on FTDNA or GEDMatch (with everyone) and on 23andMe (with your match's approval). Still, lack of ability to compare matches and quantitative data has been AncestryDNA's biggest shortcoming, and the fact that they are adding any functionality at all without requiring a paid subscription (like they do with their "DNA circles" currently in beta testing) is a pleasant surprise.
Bear in mind that just because you share a match with a relative doesn't mean that you are all related the same way on the same branch of your family tree. Especially if you're like me and come from old American families that did a lot of intermarrying in the last few hundred years. BUT if you've got a close relative on there such as a parent, you can now use that information to tell if someone isn't on their side of the family tree. Not a match with Mom? Must be a relative from your dad's side. If you're looking for an anonymous parent and you get your other parent -- or failing that, a half-sibling or an aunt, uncle, or first cousin -- to test as well, this can help narrow things down enormously.
One of the best things about AncestryDNA is how easily they link to user family trees. The trees on Ancestry are more reliably filled out and easier to view than on 23andMe or FTDNA or GEDMatch. AncestryDNA also seems to appeal more as a company to the family genealogists and people who compile massive family trees. I understand how I'm related to more of my matches on AncestryDNA than on any other DNA site, mostly because of the linked family tree function and an abundance of users who enjoy building their own family trees. If AncestryDNA were to add quantitative data that allows users to view their matches' DNA and compare it with other matches like you can on the other sites, they would quickly become the best option in DNA testing. For now though, if you're trying to deduce the identity of a parent, quantitative data is key, and I recommend starting with 23andMe.