How can I accept or ignore all Facebook friend request at once?

Yes and No :O(

Unfortunately you can't accept all friend requests at once but you can ignore all of them. I think the reason behind this is because of the privacy settings everyone has on their profile. You might not want all these friends to have the same access to your profile therefore you need to add them one by one. If you think about it, it makes sense, right?

To ignore all friend request:

1. Click on x friend requests (x=number of friend request) or See All link on the top right corner of the Home page.
2. Click on Ignore All link on the top right corner.
By the way if you don't see anything happen when you click the link make sure to either hold down the CTRL key as you click or allow pop-up blockers.
3. Checkmark the box next to Friend requests (x), and then click Ignore these requests.
4. Click Okay.

In step 3 you can also see other requests sent to you by your friends and groups so just make sure you checkmark the correct thing.

Hope this helps!