Every 8 hours, the game provides you with a list of 6 random players that you can invite to become your friend in the game. If you are active in the game when the 8 hour window expires, then you will immediately receive a new list of 6 random players and a new 8 hour window begins. If you are not active when the 8 hours expire, then you will receive 6 random players and a new 8 hour window the next time you start to play the game.
Until recently, virtually all of these random players were ones who had not been active in the game for extended periods of time (sometimes over 2 years!). The prevailing wisdom was that it was a waste of time to send friend invitations to these random players. On the flip side, if you were an active player, you would rarely appear in the list of random players to invite for other active players.
A recent update to the game changed this feature such that now the random players in your invite list are all active players (this is from my own observations during the past several days). Once again, on the flip side, if you are an active player, then you would appear more frequently in other active player's invite lists, resulting in an increase in your friend invitations from other players.
A Peek Under the Hood – How it Works
When a list of 6 random players is generated for you, there is a time stamp provided for each of them which indicates the last time the player’s game app communicated with a game server. There is also a common time stamp which indicates the time when all 6 random players were selected for your list (the beginning of the 8 hour period). All of the information for the 6 random players (including time stamps) is a snapshot in time, taken when they were selected for your list and is not updated during the 8 hours that they appear in your list.
The units for all of the time stamps is seconds. For those who would like to know more about the standard for the time stamp system used, search for either Unix time or Epoch time. Basically, it is the number of seconds that have elapsed since Unix Epoch (January 1st, 1970 at 12:00 AM (00:00:00) UTC).
Simply put, subtracting a random player’s time stamp from the common time stamp gives the number of seconds between the last time their game communicated with a server and the time they were selected for your random player list. For example, if the difference between the common time stamp and a random player’s time stamp was 248 (seconds), this would mean that their game communicated with a server 4 minutes and 8 seconds before they were selected for your list.
This same time stamp system is used for your friends list, which is why you have the “sort by activity” option for your friends list.