how do i add a faecbook account

Yes, it is impossible to link your Battlelog to your Facebook account in Battlefield 3.  However, it is very easy to do so in Battlefield 4.

1. While signed into Battlelog, scroll over your name in the top right corner.

2. Click "Settings"

3. Click the "Privacy" Tab

4. The last option is "Facebook Sharing", so click the slide tab.

5. A pop-up window will appear. Click "Enable"

6. Another window will pop-up. Here you can either sign into your profile, or it may be linked already, and you just have to click "Play Now" in the bottom right corner.

7. Boom...Done