Ideally, by the time you complete this task, it has been a couple days since you updated your work status to include Knack (and made your profile link easily accessible), making it the perfect time to post a more formal announcement about your tutoring abilities.
You never know which of your social media friends, their friends, or their friends’ friends need your help. Therefore, posting on your timeline to announce that you are “Open for business”, so to speak, is a great way to get the word out to all of your extended connections.
Posting to your own social media is a tool that you can use as often as you please and are comfortable with. That being said, we would advise you to limit these posts to the start of the term, before midterms, and before finals so as to not alienate your network.
Networking? Good.
Spamming? Not so much.