Based on your post, it's used more than you think. Simply for all formal occasions and meeting strangers. But for instance shop employees can address you with je if you're under 30 (and they are roughly the same age).
When in doubt, use the formal u. If it's too formal, they'll tell you to switch and all is fine. If it's the other way around it's quite awkward.
This is a response to your last comment below. Well, the subject of media influence reflects my own extensive study and work in life. There is a lot that is understood about it, but of course it is best appreciated with an awareness of the disciplines, say, of psychology, sociology, and its subdisciplines. As a simple observation, anthropologist Richard Robbins has written about the role of a phenomenon related to language development in the US, advertising. It has a history, and has gone beyond the US to affect the world, of course. It's no accident that everybody knows the name of Coca-Cola and Pepsi, for example. Those brand name words have been propelled by those companies, and this has affected local cultures. I was happy to see an exception, for example, as I visited health food stores and discovered a few alternative brand names that few people talk about. The Slow Food movement from Italy has been another, related response to the basic phenomenon in question. That gets into diverse and interesting subject matter. As for Belgian Dutch, the basic information that TV shows ARE exchanged and subtitles are used indicates a certain preservation until now of differences between the groups, which is noteworthy to someone like myself. Anyway, cheers, and here's to Slow Food in Dutch, Langzame Eten?
Just took a look at a Dutch Flemish discussion, and learned that ge/gij is an old form preserved currently (vs. je/jij) and that subtitles are used for informal talk in Tv shows. It seems that modern media hasn't had the same degree of impact on the Dutch as it has on American English
Ge/gij is only used in Belgium. And I think you cannot make statements about the degree of impact of media has on different languages based on one example. Both the impact of the media on language and how languages as a whole evolve are very complex matters with various aspects, not easy (and I guess often impossible) to determine cause and effect.
The word alstublieft is a contraction of als het u belieft ("if it pleases you"), so the 't' probably comes from the "het" in that phrase. Similarly, alsjeblieft comes from als het je belieft.
In Afrikaans, the word has been simplified to asseblief. French uses the same phrase, but it has not been contracted to a single word: s'il vous plaît ("if it pleases you").
Something I noticed a number of times in Belgium, particularly West Flanders, when I was there in October was how alsjeblieft was used 1. when I was buying something in a shop, and my groceries were going through the conveyer belt, a bit like "here you are," or "if you please, come through" and 2. as a response to my "dank u wel." They seemed to be using it a lot like the French use "je vous en prie," or the English archaic "I pray you." - a kind of "please, don't mention it," that I found very charming. I'd love to get this confirmed with more experience, or by a West Flemish Belgian. I was curious also to hear the alsjeblieft form instead of the alstublieft form from strangers.