I did lose a great many friends whenever I moved from one place to another. Usually meeting up once-twice a month keeps the thread alive, but closest people will stay in touch online probably forever.

That being said, if you put some work into it, you will find new friends. This requires a mindset of getting involved in any open event that you're even slightly interested in. The bigger the town, the easier it seems to come across those.

In the other hand, talking to neighbors is easier when there's no one else around. I have an impression this creates a tighter knit community, where you can find new long-term friends.

This would probably happen in my case if I didn't relocate again :)