Rare countries

So I've got my Postcrossing settings so that I'll never have two cards travelling to the same country at the same time. You can choose to do so, but I like to get as many countries as possible on my list, so I have this option turn off for now.
Last weekend, as I was requesting addresses, I somehow got a Russian address even though I already had a card travelling to Russia. Next there was Germany, Belarus, the Ukraine, US, and China. Some of the most popular and populous Postcrossing countries (apart from the Netherlands, which I choose not to exchange any cards with). So I sent a message to Postcrossing support, and they explained:

In Postcrossing there is no guarantee that you will always be sending to different countries at the same time. This would be an impossible guarantee to offer as, at some point, there just wouldn't be enough countries with enough active members to provide this. What our system does however, is try to maximize the number of different countries you are sending to at any given moment - same thing for the countries you receive from. At some point, repetitions must occur, or you (and everyone in fairly large and active countries) would have to be sending to smaller countries where there just aren't enough users to be selected. If did this, those users would receive postcards in much higher than amounts then they actually send.

We realize that many members prefer variety and our system does its best to offer that when possible. The way repetitions will happen is that they will be more frequent to users who are sending larger amounts of postcards at the same time, while repetitions will be less common for those who can send less. Since those who can send more at the same time have, by consequence, a wider variety of different countries than those who can send less, we feel this is more fair for all.

Another requirement for repetitions to happen is that, otherwise, members in larger more active countries would not get enough postcards back. They would be sending, but not receiving back. So repetitions are an actual requirement and our system aims at keep things as balanced as possible.

Lastly, I should note that the level of repetitions can and does change overtime. Our system automatically adapts to the balance level at any give moment. And with this said, I can tell that we are currently are experiencing an unusually large unbalance between countries with a large set of members in some countries not getting enough postcards (they have sent, but they have not yet received). This is probably due the mail backlog that is usually generated over Christmas and other conditions we do not control. And, for the reasons I explained before, this is why you might be seeing seeing more repetitions than usual. Lastly, to be clear, repetitions will always occur at some point in Postcrossing - what might differ is when. The more postcards you send, the higher chances you might get repetitions.

So I realised that up until this point, I'd actually been pretty lucky not to get repetitions.

Also, I'd been quite lucky to get an address in Georgia (the country, not the state), which I'd never exchanged cards with, just before I got the repeat addresses. But this got me wondering: is Georgia really such a rare country, or was it just (bad) luck that I didn't get them before this?

Postcrossing keeps a list of all countries in the program, some of which are not technically recognised as countries but counted separately anyway. There are many many countries with 0 or 1 member, including the Vatican, and some fewer with 2-10 members, including exotic places like Syria, Afghanistan and North Korea.
Safe to say, I haven't been receiving cards from any of those places. However, I did get a card from the Bahamas, which has only 12 members and sent only 1,012 cards. I also got a card from Egypt (followed by a surprise Christmas card), which has 73 members, and Vietnam, which has 77, but that's it as far as countries-below-100-members goes. Still, pretty nice.

As for sending cards, my rarest card has gone to French Polynesia, which has only 10 members, followed by Uzbekistan which has 98. The card to Georgia is currently travelling, but when it arrives it will fall in this category, as Georgia has 23 members.

In total, I have exchanged cards with 64 countries, out of the 217 participating in Postcrossing. Some of these I have strange statistics with, such as receiving 5 from Slovenia but never sending a card there, or sending 10 to Turkey but only getting 1 back. There are some countries I would really like to receive a card from that aren't that rare, such as Iceland or Denmark, and then of course some of the more exotic places, like Nepal or some of the Middle Eastern or African countries. But on the whole, looking at these statistics, I think I've done pretty well so far. It's logical that most of my cards will be going to Russia, Germany, the US and China, but apart from these, there have been some very nice surprises.