Would Wikipedia still be as popularly used if they were a for-profit advertising funded company?

There's a difference between for-profit and advertising-supported. WikiHow is a non-profit but advertising-funded wiki. The now dying and soon-to-be-reborn-on-Wikimedia-Foundation-servers Wikitravel is both for-profit and advertising-supported. I think in general, people prefer supporting a non-profit site. The idea of people spending countless hours writing articles and doing administrative and maintenance work for free knowing that someone else is turning a buck off of it isn't particularly attractive. That's different from advertising supported. WikiHow contributors seem okay with the advertising. Wikipedia users are rather more caring about that. If advertising were introduced, many would leave immediately and never return. Same for readers, possibly.

If it would have the same content - yes, it would probably be as popular. Most people link to Wikipedia because it is useful for defining stuff. I am really not sure that it would have the same content, however, because advertising would definitely influence it. Advertisers would try to skew articles about topics that are important to them. Chick-Fil-A would try to skew articles about KFC and homosexuality. Diageo would try to skew articles about alcoholism. Car manufacturers would try to skew the article about Ralph Nader. And most likely they would succeed at skewing them. So the content wouldn't be the same, and the public would have more reasons to suspect Wikipedia and less reasons to love and respect it.

I think that Wikipedia is so popular because it has such a rich & diverse source of people updating it. As well as having a neutral point of view policy (NPOV).   Because it is not for profit, it makes use of a large quantity of volunteers, (a profit based company could not afford all these people) and as a consequence of this its diversity of knowledge makes it so popular (or maybe it was planned this way).