cross-friend
The act of getting two of your Facebook friends from unrelated social circles to "friend" one another based on compatibility and/or complimentary facets of their personalities for their mutual benefit/support. Typically accomplished through friend suggestions to both friends for the other, or both friends interacting with your Facebook page and friending one another. Derived from the term, cross-pollinate.
My best friend from college and one of my co-workers would really hit it off. I should cross-friend them.
cross-friending
Cross-friending is the practice of becoming friends on social media site with a friend of a friend, without your immediate friend's knowledge. This is considered poor etiquette by many.
When my co-worker cross-friended my prom-date from high school, I told him cross-friending is usually a bad idea.