"Dropping a line" is a phrase commonly used to refer to the act of writing or sending a quick message to someone. However, there are several other synonyms that can be used to convey the same idea. Some of these include "sending a note", "penning a message", "writing a quick update", "sending a short memo", "firing off an email" or "kicking off a message". Depending on the context and urgency of the message, any of these phrases can be used effectively to communicate the same idea as "dropping a line". So next time you need to communicate with someone quickly, consider using one of these synonyms to add some variation to your language.