pixel_player45235549 - Adding friends in pixel doesn't add them as neighbours. Or at least not currently, whether it's changed in the future is down to the devs.
If you are friends in facebook and both play then you need to sync the game through the facebook game app (not through pixel federation)
Search in facebook for the diggy's game page and play the game there and all your facebook friends that also play the game will automatically appear as neighbours.
You can then go back and play through the pixel federation page if you prefer.
However you will need to resync the game through facebook again if you add new friends.
Plus in the facebook app game is the only place that the invite button in the neighbours section appears (see picture at top) it doesn't appear through pixel federation app. And this only sends an invite to your facebook friends that don't currently play the game. which could possible annoy them if they get loads of invites and they may complain to you, shout at you or unfriend you.
And this invite button doesn't seem to work for your recruiter achievement. I invited a facebook friend to play through this button, they started playing and are now in my neighbour section as level 76, but still my recruiter achievement is still at level 1, 0/5. so I haven't used it since. and I have over 300 neighbours so I don't know how you are supposed to get that achievement.
