How to tell someone you don't want to add them on facebook?

[an Imaginary cat owner]

they saw you opening facebook on your tablet/computer and ask you what is your facebook name so I can add you.
and you really don't want to share your virtual life with a family member/ co-worker/ friend

[?]

For me i say " sorry i am not adding people right now"
But i will think of more

[pat z]

You could tell them your Facebook account is really to keep in touch with a very small number of family and (distant) friends, that you're just not into accumulating as many "friends" as possible on your pages and that you prefer to communicate with them face to face rather than through Facebook. Then simply ignore their friend request.

[flowergirl]

Say something like, 'Sure, it is xxxx. I am not really into facebook you know. Just happened to check it right now, after like, six weeks! I also don't like adding random people and acquaintances as friends. I only ever add my family and some really close friends'. Then go and block them and they'll never be able to find you. And if they do, you've already said you are not really into it so they will think you just haven't checked it.

An excellent feature of FB is the 'restricted' setting. Add them and put them in the restricted group. That way they'll only see posts that are public, ie, only your cover picture. This way you would've added them and at the same time not compromised on sharing your personal life details. I use this setting for all office colleagues as you don't want to hurt them by not adding them. This will give them no chance to complain.