Does the oath apply only to Instagram, or does it include everything similar to it, based on my statement: "I swore not to message via Instagram, and everything similar to it"?

The oath includes everything you intended, and righteousness dictates refraining from messaging via the applications you intended. If that becomes difficult for you and you need to message, then you must pay an expiation for breaking an oath: feeding ten poor persons, or clothing them, or freeing a slave; and if you cannot find [the means], then fasting for three days.