Engagement shoots are a great opportunity to get photos of you and your partner that are professional and intimate. Your engagement photos can and will be used for a number of things, but if you were born after 1980, it's likely that the first thing you'll think to do with them is post one or two on social media. The only downside to having so many gorgeous photos from your engagement session is that now you have up to 100 photos to choose from (whereas before you probably would've had one or two professional photos from other people's weddings you could've used).
If you were lucky enough to get a professional photographer to shoot your proposal (or if you get your engagement photographer to shoot a reenactment of the proposal--in 10 years it won't matter that the photos weren't from the actual day), this is a natural choice. You don't even have to include a caption for people to understand what your post is about. If someone else took pictures during your proposal or you took a selfie afterward to text to your best friend, this photo might be worth including along with one of your professional shots. If your Instagram account is highly curated, you might not want to post a grainy photo taken in a dark restaurant or a photo of you crying or laughing hysterically. It's always nice to have a proposal photo, but it's okay if you want to keep it just for you.
If you come up with a beautiful and meaningful caption for your engagement announcement and one or two of your photos instantly fits that sentiment? Perfect. Go with that. The above photo could be captioned with something about NYC whether it's about starting a life together in Brooklyn or about where they first met in Manhattan. If your engagement shoot is done in a place that's meaningful to you, which I hope it is, your photos will be begging to be captioned with the place's significance in your relationship: be it the place you first met, where you went on your first date, or your favorite spot to hang out together. Falling in love happens internally, but it also happens in places. Use place as a determining factor for where you should have your engagement shoot and tell your audience of social media followers about the place's significance in your eyes.