I participate in myheritage.com. It has a feature called smartmatches that show you where they surmise other trees have common ancestors with your tree. I find this feature both useful and annoying. I try to 'confirm' the smartmatches (their term for when you acknowledge they are the same people) for people I think actually share those relatives. For example myheritage tells you your relation to the person and their relation to that person.
The annoying part is distinguishing people with common ancestors from people trolling to make their tree bigger. I have 3,023 matching trees at the moment and a large number of them are of people with trees in the tens of thousands (some hundreds of thousands). Many of the creators of these trees have multiple trees of this size and the trees have names that seem to me are computer generated. They don't appear to be related to the people in the tree.
Is there an easy way to sort the smartmatches by size of tree so I could focus on smaller trees? I can only sort on number of matches which is loosely coupled to size of tree. However, doing this I missed my 2nd-cousins tree (which was small but had a large amount of matches); yet I still got lots of huge trees that only had a single match.
Note one of my concerns is confirming those single matches (which I did early on) leads some of the people to then fan out from that match and include my entire tree in their tree. I keep my tree private and include my living relatives, so I was very upset when this occurred since I promised my relatives I would not expose them to non-relatives which is why I am more careful now.