Google Analytics says the source of most of my visitors is social-buttons.com. How are these visitors finding my site?
This kind of visits are called Ghost Referrer Spam since they never reach your site. They use a GA weakness to make a fake visit and get a record in your data.
They do it to get traffic, people get curious to see who is visiting them and click on the link.
This specific Referrer Spam is nasty because it make multiple visits at the same time, is related to the number of the subdomain so if it says site38... it hits with 38 visits, I've also have many of these, here is a screenshot I took:
In my case is a different simple-share-buttons.com but is the same thing.
The easiest way to stop it is by making a filter for each spammer in your GA. Check this article to find more detailed information http://www.ohow.co/block-social-buttons-simple-share-buttons-referral/
As an alternative, you can make a more general filter to take care once and for all of all the Spammers by making a list of Valid Hostnames, this is more advanced and you have to be more careful. You can find more information about this solution here https://stackoverflow.com/a/28354319/3197362
It's actually referral spam. Take a look at this https://www.mooresoftwareservices.com/Web-Commerce/social-buttons-com-referrer-spam
So unfortunately they are not good quality hits.