(1) The number of hops is O(log(n)) but, as the data is stored in several places, you have always the possibility to find a node closer to your XOR situation. The O(log(n)) is the worst situation.
Some number…
10.000 Users → 14 Hops Maximum
100.000 Users → 17 Hops Maximum
1.000.000 Users → 20 Hops Maximum
10.000.000 Users → 24 Hops Maximum
100.000.000 Users → 27 Hops Maximum
1.000.000.000 Users → 30 Hops Maximum
(2) Read that:
(3)(4)This is discussed many times in the forum. And, as you can read, my opinion is totally contrary to @janitor .
The stored data follows, the last decades, an exponential growth. While that continues, and nothing indicates will not, the new data will always be the majority. The storage cost, of old data, decreases very rapidly to become, in a few years, irrelevant.