While the Like Button allows users to share content with all of their friends, the Send Button allows them to send a private message to just a few friends.
In addition to the URL specified in the send button, you can also include a title, image, and short description of the link, and can specify what is shown for the title, image, and description by using Open Graph meta tags.
Get a combined Like and Send buttonAccording to Facebook, "you can implement the send button on its own, but if you also want a Like button on your site, you can simply add
send=trueas an attribute to the Like button. This will render the Like button and Send button next to each other."
For further information, see the documentation on the Like button page.