what is the best practice www.domain.com or http://domain.com or http://www.domain.com to give for backlinks??
It depends how your site is setup..... www.domain.com is not a good way... Be sure to either forward your www. to your non-www. or forward your non-www. to your www. Look up "301 redirects " on google for how to do this. When link building, use http:// in your links, and use whichever one your site uses. Here's a writeup about it.... http://courtneytuttle.com/2007/07/27/www-vs-non-www/
From my experience it doesn't make a huge difference, but there are guys that will tell you it does... Pretty much just choose one variation and stick with it, either http://www.site. or http://site. Then just build the links to which ever you choose and use a redirect so when people link to the one you didn't choose it gets forwarded to the one you did... http://www.google.com/search?q=htaccess+redirect+301
They are the same, make sure you use only one version when building link or you may use google webmaster tools to claim your prefer version.