It appears to me from my own personal experience that hostesses often do not receive a response to their invitation to attend their parties. This could only mean one of two things, either rudeness is a growing trend in modern society or people are ignorant to the meaning of R.S.V.P.
R.S.V.P. comes from the French expression ‘respondez s’il’vous plait’ which means in plain English’ respond please’. Any good hostess needs to know numbers of people who are attending in order to cater for food and drinks and considering you have been privileged to be invited to their home or party the least you can do is respond either formally by card or informally by telephone.
To R.S.V.P to your hostess is simple good manners, therefore, next time you receive an invitation please call your hostess and respond promptly regardless of whether you can or cannot attend.
So how did R.S.V.P. make its way into being used for wedding invitations and the like? This dates all the way back to around the 11th century when French became the fashion among the elite of the English court. This continued in England for several hundred years. It was also the fashion in the United States, among high society, to use French as the language of refinement up until around the 19th century. From this, many such similarly themed French phrases and words made their way into English, another example being the word “etiquette” itself.
Ironically, the French themselves don’t usually use R.S.V.P. anymore, more typically using Prière de Répondre these days.