A recent study reported here claims that multivitamins do not have many of the healthful effects their manufacturers claim. This news will probably not lead to a significant decrease in consumption of multivitamins for the following reasons. First, by and large, at least according to the yahoo report I linked to, multivitamins haven't many adverse effects. In other words, taking them will not (as far as the study knows) harm people.
Second, and related to the first reason, the cost of taking multivitamins is significantly low in relationship to the possible benefit of taking them. The multivitamins I buy cost less than $20 for more than a year's supply. The additional, non-monetary cost--such as having to sacrifice the five seconds each morning it takes for me to swallow the multivitamin--is negligible.
In other words, consumers will probably say "it's relatively cheap, it's (probably) not bad for me, and it might just be good for me."