To keep in touch with ...

Now I am writing a mail to friends of mine in foreign countries.
It is the year-end, so it is the season of Christmas cards.

Most of them live apart, not in Japan but abroad.
So it is a little difficult to keep in touch with them.
They are also very busy and I am also busy,
so we often forget to reply to each mail,
and the relation often finishes unfortunately.

Then, not to finish the relation,
At least I intend to write a mail every year.

They live abroad, so it isn't easy to come to or visit Japan.
I also can't go abroad easily to visit them.
The distance in the relationship is big.

However, I want to keep in touch with friends of mine
in foreign countries, because they are my treasure.

How do you keep in touch with your friends in far places?