The Mashable social sharing sharing button seems to be custom coded (might need to check with some experts here).
If you are not keen on using ShareThis.com, you can give DiggDigg a try. Personally, I like how DiggDigg presents its social buttons because it stays persistent on the sidebar following the visitors wherever they scroll.
Such type of buttons appear after customization in coding. You can find plugin to add into your website if you have wordpress platform. this will be easy fro wordpress users.
There are various plugins which you can use to implement this but remember that the number of likes/shares which appear on the feed are the same which are there on the facebook fanpage.
They will not be shown as 1.5k…
You can check the following links to get more info
Check this out - https://developers.facebook.com/docs/reference/plugins/like/ when you change the layout style to “box count” you will get the style you want. It is a style that Facebook provides so it should be as an option in any Facebook plugin you use.