http://twitter.com/share?text=<TITLE>&url=<URL>
E.g. http://twitter.com/share?text=This+is+google+a+search+engine&url=https%3A%2F%2Fwww.google.com
http://www.facebook.com/sharer.php?u=<URL>&p[title]=<TITLE>
E.g. http://www.facebook.com/sharer.php?u=https%3A%2F%2Fwww.google.com%[title]=This+Is%2C+Google+a+search+engine
http://pinterest.com/pin/create/button/?url=<URL>&description=<TITLE>
E.g http://pinterest.com/pin/create/button/?url=https%3A%2F%2Fwww.google.com&description=This+is+google+a+search+engine
Twitter are using Tweet Web Intent now.
Share? will still redirect automatically, but if you want to set up the URL, it'd look something like this:
https://twitter.com/intent/tweet?text=This%20is%20google%20a%20search%20engine&hashtags=searchengine,google&url=https%3A%2F%2Fwww.google.com&via=google
https://dev.twitter.com/web/tweet-button/web-intent