Question: I consider Facebook to have information that is personal and that I only share with close friends and family. However, in my new workplace I’ve been asked several times for my Facebook username and/or have been *friend requested* and I don’t want to sound rude by telling them that I don't want to add them as friends. 

Any suggestions on the way I should handle this?

Answer: 
My policy, that has served me well, is that I never, under any circumstance, add coworkers to my Facebook "Friends List" until I have left the job.

I can't see any good coming out of adding anyone while we are still coworkers. I don't need anyone monitoring my every move and I don't like having to think too hard about who is in what group when I am sharing opinions and personal thoughts. 

What if I decide to post that I'm heading to an interview (which I do occasionally just to test the waters) or if another friend asks me about a recent interview experienc? 

Can everyone I work with be trusted with that info? Even if one person can be trusted, they are like a vector by which another (untrustworthy/gossipping) employee could gain access to information I provide. 

What happens when a nosy or hostile coworker steals a glance at their phone FB wall while they are distracted etc.

No coworkers...ever...this works for me since I tend to change jobs every 2-3 years anyhow. While I'm at a job, if we really are friends, then we probably don't communicate primarily through FB. If we are friends, we will have each others phone numbers and can text/call/im/email to arrange pretty much anything.
> 
> I only connect with *very close* friends and family on Facebook.
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> 

Sounds acceptable to me.

You could even preface that with:

> 
> It's not that I don't think of us as friends, it's just that...
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> 

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An alternative (similar to [HLGEM's answer](https://workplace.stackexchange.com/a/25827/8234)), is to simply say:

> 
> I prefer to keep my work and personal life separate.
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> 

HLGEM's suggested compromise of connecting on LinkedIn, if you have a profile, is a good addition - this prevents you from outright declining (which might not come across particularly well), and prevents pretty much any nagging or arguments from them.