Facebook: https://www.facebook.com/sharer/sharer.php?u=http%3A%2F%2Fex...
Twitter: https://twitter.com/intent/tweet?text=My+tweet+http%3A%2F%2F...
Google+: https://plus.google.com/share?url=http%3A%2F%2Fexample.com
LinkedIn: http://www.linkedin.com/shareArticle?mini=true&url=http:...
But later on they backpaddled and made an offical statement that you are allowed to use an modified like button that does not resemble the orginal like button (icon).
Anecdotally, users seem to trust native button more. If it looks like you made your own PNG and linked it to Facebook, they can't trust that you aren't going to try to do something shady to their Facebook account (or open a popup). If you use Facebook's button, it's a familiar experience without providing the publisher's site the opportunity to play man-in-the-middle.
User perception matters. Facebook's button seems to be perceived as more user-friendly than the unknown button on someone else's site, even if the unfamiliar button was designed to be less hostile than Like.
I expect that Schneier's blog is one of the best places to start reversing that common (and understandable) perception. Given the pervasive nature of fraudulent sites focusing on Facebook, one can't blame the users--but shifting their frame of mind from "fraud is bad!" to "tracking/other invasions of privacy are bad!" is overdue.