How do you find people?

An interesting problem has arisen recently and it seems worthy of raising in my witterings.
Last week an employee of an executor company contacted me and asked me if I had contact addresses for a number of people named in the will of my late friend.
No. We have not been able to find an up to date address book. I have one which is more than thirty years old. Things have changed greatly in thirty years. At least two of the people named were not even born then.
By checking the phone book in another country I found one address was still current - as I thought it was - and I sent that to the executor. It was relatively easy but I had met the person in question when she came here last year.
The other people I have not met. I know what their relationship is to my friend and a tiny amount about them. My friend was not the sort of person to talk much about other people. She was much more interested in ideas.
But I did a little research. It is something the executor company employee could do and she may well have search resources I do not have. I also have resources she will not have.
There is also the internet. People in their twenties and thirties are very likely to have an internet presence of some sort and that can lead to other places.
After some searching yesterday - something which took about an hour of my time - I sent off e-mails to two academic institutions where two of the legatees are likely to have been students. My requests were discreet and I carefully backed them up with sufficient information and the offer of a reference with respect to myself. If they are, as I suspect, old scholars then I think the institutions in question will help - if they have addresses for their old scholars. That should also put us in touch with the siblings of these people.  
It's a nicer approach than trying to get access to addresses through other means - such as an advertisement in papers in foreign countries which may or may not produce a result. Even so it is a hit and miss approach, particularly as I had to send one e-mail in French and my French is, to say the least, not good. I never studied the language. I read some French but I don't write it.
So, we will wait and see if my requests bring about any results. I hope they do but it occurs to me that all of this could have been avoided if we had found an up to date address book.
I had better up date mine.