"I think most Americans stopped using butter about 30 years ago - when it became publicized that animal fat caused high cholesterol which contributes to heart attacks. Margarine used to be a POOR substitute for butter and was only used by people who couldn't afford the real thing. It used to come with a squeeze packet of yellow dye that you'd add to make it look like butter (it was white, otherwise). That was before my time, though, like in the 1940s during WWII when luxury items like meat and butter were rationed.
Somewhere along the line, in the late 60s, I think, people started paying attention to their health and their diets. They wanted something that would substitute for butter that would be healthier. Technology advanced and margarine improved to the point where most people couldn't really tell the difference between that and butter and those (like me) who could tell the difference, liked the margarine better.
When I bake cookies, I buy margarine in the sticks. The consistency works better for baking than the soft margarine does. Otherwise, there's no difference. My dad, however, still prefers the taste of butter and uses it on toast and other things where the taste is prevalent and a substitute won't do. He uses margarine for cooking and in situations where the taste will be masked by the other ingredients. Jim (Jana's roommate) says "but butter tastes better" but in reality, he can't tell the difference.
Vegetable products like Crisco and Mazola oil were introduced in the early 1900s, but didn't really catch on until the 1960s. First, we stopped using animal fat for frying at home, then later the fast-food industry switched from animal fat to vegetable-based oils for frying. That happened in 1985-1986.
I remember my Grandma being really upset when she could no longer buy lard at the grocery store. That must have been in the 1960s, since that was the time my Mom went back to work and I started staying with Grandma more. My Dad also took over her grocery shopping duties at about that time and I went with him on Saturday mornings. We bought her that first can of Crisco and I watched as she used it for making pie crust. She complained the whole time, but it turned out just fine. An added plus was that the Crisco didn't have to be refrigerated like lard did. I never heard her complain about the unavailability of lard again.
I'm sure that's more than you ever wanted to hear about margarine!"