Hello you wiser and more experienced Postcrossers!
I have only just started and my first cards have reached their destination. 
So, now I have been receiving Thank you -notes ( 
), but I am not sure whether I should react to them or not.
Do you expect to get a message back after sending a thank you? Or do you reply to them always/occasionally/never?
Wonders, Schnappi_72 aka Soile
It’s very rare to receive an answer to a Hooray Message. But if you feel like you want to add something, go on. But don’t feel like it is expected!
I’d say the huge majority of thank-you-messages is one-way. There may develop some follow-up exchange sometimes, but in my experience, it’s the exception.
Personally, I guess I have had ~200 extra messages of overall ~4000 sent/received, ranging between answering one question to some thoughts about mentioned books to the start of further exchange of postcards.
To leave it more open and complicated: it depends 
(but as said it’s more exception than standard)
I don’t expect the communication continue after thank you message, and secretly I am hoping it really wouldn’t continue further from there as I really suck with online conversations. So far that I have experienced, nobody replies to thank you messages. Unless there is a questions or trade offer or something like that, of course. I have myself so far replied for two thank you messages, once because they asked for more information about the stamp I had on card and once because they were interested on trading cards with me.
So I would say, unless they seems to want to keep the conversation going and you want that too, then reply, but otherwise you don’t need to.
I’ve just started getting Hurray messages too!
2/3 had questions so I replied. That’s all tho haha.
I’m always replying them.It’s good to know what people thought about my postcard.
I always replay to Hurray messages. I’m always happy that my card arrived and I want to express that and wish people a nice day or whatever.
I only started recently and already had a feeling that it wasn’t very common to react to an Hurray message, since I haven’t received one myself. 
Sometimes I get a spontaneous response to a Hurray message and sometimes I respond to a Hurray message. The reactions to each other’s answers have resulted in pen-friendships, which is nice, especially since the contact was unexpected and spontaneous.
If there are questions or something that the sender wants to talk more about, I send out messages hihi 
I gained friends for years doing this
I answer them if someone asks me something but otherwise I consider the postcard/hurray message the completion of our brief interaction in the world 
Sometimes I get a spontaneous response to a Hurray message and sometimes I respond to a Hurray message.
I guess this will be my approach. If there is a spontaneous reply on the tip of my fingers, I will go for it. Otherwise, I’ll just send happy thoughts… 
I sometimes ask questions in my registration message and in those cases I hope to get a reply.
And sometimes I reply to to the Hurray!-Messages, particularly if I think it is expected.
That’s pretty much what I do. Sometimes I receive a great thank you message and I feel like replying but I don’t have time, I’m on the go etc. and then I forget or it doesn’t seem relevant anymore.
Also, because the message from the website goes into your email, I always wonder if people will receive it or it will go into their spam.
I had a few conversations but generally I agree that it’s rare.
I reply if they ask a question but that’s it.
It generally isn’t expected, I myself only reply when someone asks if they might send me a postcard in return for my card (some people like doing that), or when someone wishes to swap with me, or asks me a question or something. One time someone was confused about my user name not matching my actual name and they thought it was someone who had written down the wrong postcrossing ID and that they had registered a card from someone with an ID actually meant for someone else’s postcard or something, so I had to explain to them that my username is not my actual name and they had in fact received the postcard with the correct ID. (Nice of them to ask, I was pretty surprised when I received the message asking that though.)
The only time I ever reply is when people leave long thank you messages with questions in them! The last time I replied it was when someone asked me who my favorite character was in a show that we both liked.
Usually the conversations stop there, unless there is something to follow up on, such as a trade or them asking for more information about the place featured in the postcard or the stamp.
I used this stamp on a postcard before which showcased the BTO (“built to order”) process in Singapore, and I briefly explained that this is the process all married couples have to go through in Singapore, which is to submit a bid for an apartment (given our land scarcity) to live in after marriage. The Postcrosser asked a few questions back as he was curious, e.g. what happens to the apartment if the couple ends up not getting married.
Otherwise, I think it is completely fine not to reply or expect a reply from the Postcrosser. 
Sorry for the stupid question, I have only sent cards and not received any yet:
How do “Hooray messages” work?
I got one - I saw it only in my email, there’s no inbox in the Postcrossing page. How can I reply? I assume I have to enter the other person’s profile and send the reply from there via the message button? Or can I reply to the Hooray email?
Is it the same way to send Hooray when I receive a postcard - user profile —> message button?
When you register a card, you type in the ID-code and write the message below it. There is a place for the message, which will be a part of the Hurray mail. But you can’t reply the Hurray mail directly, you do it via profile as you guessed.