1. in relation to the subject being discussed
speaking of David, have you seen him recently?
The expression 'speaking of...' is most commonly used for introducing a new topic into a conversation that is connected to something just, or at least very recently, mentioned. Switching from one topic to another in the middle of a conversation can seem rude or lead to confusion, but using this expression allows for a much smoother, clearer transition. It is more commonly heard in social situations, but can also be used in an informal professional context, and shares a similarity to the phrases 'by the way,' and 'in/with regard to.'