Hi keceje, thanks for reporting this issue
It's a bit disconcerting to see one's neighbours disappear all of a sudden
As of the update to 0.16, players who have been inactive for 30+ days will be deleted from the world map from now on. As no players had been removed since the beginning of the game, many players were removed this time. The 'holes' left in the map will be filled by new players or returning players.
From now on, players will be removed when they have been inactive 30+ days, so only a few will be removed at a time. You can find the announcement about this
here (click)
As this is not a bug, and was also reported
here, I'll mark this thread as a duplicate and archive it. Please do let us know if you find any other problems with the game. As the purpose of a beta server is to find and fix bugs before the new update is released on the live servers, we'd rather see something reported a dozen times (even if it does turn out not to be a bug!) than not have a problem reported at all