As someone whose always had very minimal information in his facebook account here's what I noticed about the "people you may know".
I've never used mobile facebook app on any of my phones, Ive also never given facebook my phone number despite the repeated begging so your mileage will vary.
I did enter my high school (but, i went to 2, not one) at one time and several people who went there are also on my "people you may know". Some of these people are also friends of my friends. I don't see anyone from my high school #2 that I was never able to list. I deleted this information off my profile (6 years ago), but these people are still suggested.
I also at one time listed my city where I lived. Then I had lots of "people you may know" who are also listed from the same city. Did it once for my hometown where I was born and it also suggested tons of people from that place. I also deleted this information off my profile (6 years ago), but these people are still suggested.
Also the same with the "interests", "recommended pages" based on pages I "liked".
I believe there is some algorithm present that links you with other stuff based on your hometown, where you live, your friends, school, where you work, etc. It also looks like whatever you enter, even if you change it later on will still remember your previous values.
My suggestion is if you create a closet account on facebook. DO NOT FRIEND your current friends, it will pick up on it may suggest your closet account to your other friends. This will also hold true if you enter your place you live, past schools, same interests, etc. The more you fill in data that matches your non-closet account the higher likelyhood it is to have your accounts suggested to friends.
My secondlife persona's facebook account was completely unknown until a friend of mine added him as a friend on her profile. Now my secondlife persona is on all my friends accounts.
Food for thought.