There is no built in Facebook or Twitter share button in iWeb (Although you can have your Facebook page update automatically when you update your iWeb site. Type in "Facebook" in the iWeb help menu for details).
However you can use an HTML snippet to add that kind of functionality to your site.
While I have never personally used Facebook or Twitter shares you can add their code:
That's encouraging, Madison, but how do I embed the snippet in iWeb? For example, I can only create a hyperlink to one of my pages, and external page, or an email. I don't see a "run script" or equivalent way to embed such code.
The trouble is that you paste the code into a widget HTML snippet (which is a iframe), so you share the file of the button and not your main page.
I use [addthis|http://www.addthis.com> to display buttons (facebook and twitter) in my [french website|http://iweb.debutersurmac.com/siteiweb/Blog/Blog.html]. You can try them. There's [some tricks for sharing the good page|http://www.addthis.com/forum/viewtopic.php?f=5&t=25608] if you have its URL.