njrick:
Have the reason there in your own mind, if you need to, but unless that reason advances the plot, or explains a character's motivations, leave the explanation out of the story.
Here I disagree. In general, background info which does not or need not advance the plot or directly influence or explain character motives will IMO often enhance a story and make it richer, *provided* that the story is not overwhelmed, and particularly that the background is not presented in a huge lump or "Infodump". (Of course that means that a long story can tolerate more background than a shorter one could.) GoodGulf has been adept in presenting background info in many stories, including his "County line" series. It may well be that NJR has encountered too many unskilled writers who dump background info into a story in undigestible lumps.
All that said, if you want the story to be modern, why not use email? I agree that a mix of FB, twitter, email and other media might well be too confusing. But particularly for long and private communications, basic email is more attractive anyway, and many people choose not to use social media in any case. (I do not, for one, and I am hardly digitally challenged.)
Epistolary stories can be great fun. My "Red Letters" is on this site. A favorite of mine is _Freedom and Necessity_ a novel set in the 1850s (mostly). that has four main writers, and several less frequent ones, and you have to watch who is writing to who, because some people want to conceal or distort some facts from particular others, and some are more careful observers than others. Another fun one was _Sorcery and Cecilia_ a sort of magical version of a Jane Austen novel. Yet another was "Computers DEon't Argue" in which many of the letters are computer-generated, although printed on paper, as it was a pre-internet story.
I do think that some indication of why the characters are exchanging communications is desirable, but why they choose a particular format, unless it is a plot point, need not be gone into in detail, in my view.
I look foreward to seeing this, and if you want a beta reader, let me know.