Having tried a number of different Share Button Adders, I became increasingly frustrated with the over-complicated ones I came across.
Some hotlinked to other sites and took ages to load, some had an overwhelming settings screen and many boasted a 'floating' option which just didn't work on my theme on both mobile and desktop.
So... I set out to make one for my own use and figure it'd be rude not to share it!! Perhaps I'm being biased but it's exactly what I wanted, 5 stars from me!