(idk if this applies but!) i've found that with long distance friends reaching out once again via letter is much, much easier than in via texting or social media or facetiming or whatever (phone calls can go either way). *
letters lend themselves to introspection, asking too many questions (in the best way) and offering random anecdotes/basic information all in one place rather fluidly.
+ it shows very clearly the value you find in y'all's friendship through how you make the thing! What card or kind of paper did you use, what's your handwriting look like, did you pick out a cute sticker or 2? etc. all these things from the get-go show that you want to put effort into reconnecting!
*you may need to use these methods first to pop the awkward question of "dude what's your address? (,,¬﹏¬,,) "
another caveat: people will want to put time and energy into their letters!! this has the adverse effect that sometimes it'll take awhile to get a response (˘•‿•)