In Ireland and other coastal, rocky parts of the British isles as fertilizer and a way to make a topsoil where there isn't one. So while it's not a core part of the diet, it's a crucial part of farming, and it's been this way since neolithic times according to soil analysis.
And as Falecius mentioned, seaweed is also used in meals, usually as part of oatmeal.
I don't see this really catching on except in certain geographic environments. So seaweed and algae would likely be eaten only in coastal areas, especially if the land is poor. It's not an especially glamorous dish like, say, any sort of meat, so nobody's going to bother transporting it over long distances. As a result, it'll likely remain a local food rather than a widespread one.
I'll take America as an example. If algae's part of American cuisine in the colonial era, it'll still not really spread west, since you can't grow much seaweed on the Great Plains. By the 20th Century it's considered a weird local food endemic to parts of the the East Coast. On the other hand, there's a good chance it becomes something health nuts go after.
So that's not exactly an answer, just a rephrasing of your question and a clarification of the problem.