That icon you're referring to was created by ShareThis so using it in the manner you wish would be incorrect. Reference:
A share icon was introduced in 2006, known as the Share Icon, used by
ShareThis and licensed under four licenses: the share-alike GPL and
LGPL, and the permissive BSD license and Creative Commons Attribution
2.5.5 ShareThis produces widgets for accessing social networking services from a single pop-up menu.
In regards to what Naty posted you could use that as an icon which is typically found as the share icon. Depending on how your design is made I would suggest just using the icons developed by the social media platforms you're going to use because you would just be creating another avenue to click and it would cause iteration to your users if they click the icon and found out they cannot share/like it on whatever you are doing.
Since that isn't your question I will just post some other examples:
All of the above icons can be found on Iconfinder. If you are set on using an icon to open other share icons you could always take the theme of your site, place it in the center and used the social sites minimized logo off to the sides. So for example if you have a dog website take the last image above and replace the cloud with a outline of a dog and the surrounding circles could have each individual symbols, like the F for facebook, twitter bird or T, the g+ for Google, etc. etc.
