That is a very short time frame, no need to be worried.
17 days is not that long, people don’t log in every day if they have nothing to register/send. Or they are busy with exams, or work, or something else in their life… it is really too short a time to worry.
Not registering for 3 months might just mean they didn’t receive any cards for 3 months, maybe they don’t send much. I sometimes have gaps of 1-2 months simply because I receive a lot all at once and then for ages I am not due any cards. Even though I log in more often than I should 
Sometimes people do disappear without registering, unfortunately, but you can never tell. Someone like you describe wouldn’t really worry me. Others do, but your part of the deal is to send a postcard (and I have sent to people that I thought would disappear but instead registered promptly, and to people that seemed very active and then never were seen again).
Oh and in terms of dealing with inactivity, if someone doesn’t log in for over a month (but apparently there are other factors too so this is not an exact science) their address is not given out. 17 is well within the time range. You will never get an address of someone last seen 6 months ago. But if they come back and are due cards, then you will.