Using a pseudonym I submitted my dna to Ancestry about 4 years ago. So far I've been notified of 50,497 people with shared dna matches. 4021 of them are likely within a 3rd or 4th cousin range. No previously unknowns or surprises have shown up for me, and I have not participated in any "family trees".
If someone has a real need to know if they have close dna matches (such as adoption or sperm bank matching - and don't want to accidently marry their 1/2 sibling) submitting their dna will sure reveal any relatives that person or any close relative of either person who also has submitted their dna on the same site. It would even reveal any grand or great grand children someone didn't know existed.
When I was last in a physical therapy class there was a 76 year old adopted man who was stressed out & complaining about being harassed by phone & mail by 5 people who had traced their dna, found each other, & found he is their long lost brother. He never submitted his dna or searched for them. Through Ancestry they had done their research of old records & started a "family tree". They want to meet him & are planning a family reunion, offered to pay his travel expenses, etc. He wants no part of it preferring to remember the adoptive family he has known for so many years. He wants to be left alone and is afraid one of them will show up at his doorstep since he no longer accepts their phone calls.
Some people want to know, others don't want to know at all. It boils down to a similar adage: If you don't want to know the answer, don't ask the question.
If someone has a real need to know if they have close dna matches (such as adoption or sperm bank matching - and don't want to accidently marry their 1/2 sibling) submitting their dna will sure reveal any relatives that person or any close relative of either person who also has submitted their dna on the same site. It would even reveal any grand or great grand children someone didn't know existed.
When I was last in a physical therapy class there was a 76 year old adopted man who was stressed out & complaining about being harassed by phone & mail by 5 people who had traced their dna, found each other, & found he is their long lost brother. He never submitted his dna or searched for them. Through Ancestry they had done their research of old records & started a "family tree". They want to meet him & are planning a family reunion, offered to pay his travel expenses, etc. He wants no part of it preferring to remember the adoptive family he has known for so many years. He wants to be left alone and is afraid one of them will show up at his doorstep since he no longer accepts their phone calls.
Some people want to know, others don't want to know at all. It boils down to a similar adage: If you don't want to know the answer, don't ask the question.
