Stratum or strata?

I seems to me like "stratum" is little used, in favor of "strata", even when you should use the singular, like in:

"variables like gender and socioeconomic strata are particularly appealing"

I think the correct form is "socioeconomic stratum"

What is the correct (most usual?) form?

Strata should not be used as a singular, as well as media, etc.

"Variables like gender and socioeconomic strata are particularly appealing."

I think the correct form is "socioeconomic stratum".

In the particular sentence you've quoted, without further context, it could be either the singular (stratum) or plural (strata). If you'll give us the source and some context (sentences before and after), perhaps it will appear that one or the other would be preferable. They are not interchangeable, as you seem to think.