It is very easy to add a Facebook share button using Facebook API: https://developers.facebook.com/docs/plugins/share-button/
What you need to do is just put the url in a HTML like below:
<a href="https://www.facebook.com/sharer/sharer.php?u=https%3A%2F%2Fparse.com" target="_blank">
Share on Facebook
It will launch a share dialogue.
But how about a feed dialogue? Share dialogue has a option to let user post the url and content not only on his own timeline, also can post on friend’s timeline. In some case, we don’t want this option. We only hope user post on his own timeline. That is why I will use feed dialogue.
Here is a article about the difference and implementation: Facebook Feed Dialog vs. Share Link Dialog
Everything you need is just read the above article through.