At the end of each school year, I always have a "graduation" and/or family picnic/activity with the students and their families. During that time, I provide them with an opportunity to swap contact information so that they can stay in touch and set up play dates over the summer!
Here's a quick and easy print out (just cut in thirds) and place on the tables so that you students and families can informally exchange information. If you have older students...this is something they can do on their own!
You've worked so hard to build friendships and a sense of community amongst your students---don't lose all the social gains you have worked for this school year!! Extend social opportunities into summer vacation! :)