On 16 September, 1955, Asimov wrote a letter or postcard to someone. (The one flaw in Yours, Isaac Asimov is that we usually are not told to whom Asimov was writing.)
From this letter, we may take it that by 1955 de Camp and Simak were good friends who exchanged letters...but of course that Asimov was more prompyt than they.
From pg 4
"Writing at once and answering at once is one of my many compulsions. I do not expect it of others. The pattern of my regular correspondences (as with Sprague de Camp, Cliff Simak and so on)is a letter from them, an answer from me by return mail, an answer from them any time from a month to a year later, an answer from me by return mail and so on. I virtually never write out of turn, so I end up never bothering anyone."
Yours, Isaac Asimov: A Lifetime of Letters, edited by Stanley Asimov. Doubleday, 1995