and with my experience...
The nofollow attribute generally would only be applied to an external link if the target was either unknown or not trusted.
I would not using nofollow any social sharing button link attribute if isn't like those above applied.
And I realized that ShareThis was causing my server slowing down the load times with 2 of my websites, because In the process of implementing a CSF (Security for outbound links), I was shocked to find that the total number of outbound links with ShareThis calls are too much.
a Few months ago...
I decided to replace ShareThis social sharing button tools with AddThis social sharing button tools.
So you can see, now 2 of my websites loading faster.
AddThis is better than ShareThis if you care about the API.
AddThis: Detailed api, displayed in an aesthetically pleasing way.
While with ShareThis: Detailed api, Hard-to-look-at developer page with not a whole lot on it.
Thank you AddThis.