It depends.

• What legal jurisdiction are you publishing them from as well as in what legal jurisdiction were the photos produced? Laws vary greatly from country to country.
• What type of place were you at when you took the photos? Was it a public space or private property controlled by someone other than yourself?
• What is the status of the subjects you photographed? Are they adults or legal minors? Are they publically recognized figures or private citizens? Do they have any physical disabilities or other visual features that might be considered embarrassing?
• What kind of agreement did you and the subjects make at the time the photos were taken? Did you agree, either verbally or in writing, to keep them private?

All of these factors can affect the answer to your question.

Legal Disclaimer

The following is for general information purposes only and should not be taken as legal advice for any particular situation. If you have a specific concern you should consult with an attorney familiar with the relevant issues in the jurisdiction in question. This answer assumes that the question is regarding photos taken and published in the United States.

Assuming you are in the United States, if the photos were taken in a public place where courts have consistently ruled that there is no expectation of privacy, and the photos would not be considered truthful but embarrassing then you do not need permission to use them for editorial or artistic purposes unless there is an existing contract between you and the subjects stating otherwise.

You do need permission to use them for commercial purposes, which is largely defined as implying that individually identifiable persons in the photo endorse a product or service. Note that just because a newspaper is sold by the publisher does not mean the images it contains are being used commercially. If the image is used to illustrate a news story then it is being used editorially. If the same image were being used in a commercial advertisement printed in the newspaper, it would then fall under commercial usage.

In the case of your desire to include images in your portfolio, the usage would normally be considered non-commercial or artistic. However, if you superimposed text over your subjects or in a corner of the image that imply your subjects endorse your photographic services, or you made similar comments in text accompanying the image when you published it on social media then you would need a model release from them. You could, for instance, say something like, "These images are from an engagement shoot I did in Smallville last year." You probably shouldn't caption the album or images with a statement like, "Jane and Bob say hire me to shoot your engagement session this spring!" Even if their real names are Sue and Mike, if they are recognizable in the image then that is considered an endorsement and thus commercial usage.

Beyond what is legal, though, is the question of what is ethical. If you know the subjects well and suspect they would not be happy if you published the photos then you probably shouldn't. If they broke off their engagement and are no longer planning to marry it might be considered truthful but embarrassing, which could expose you to legal liability and potential damages. If, on the other hand, you only knew the subjects casually for a brief period of time and it is unlikely that they would have connections to anyone in your social media circle then perhaps you would be in a better position to use the photos to demonstrate the quality of your work. If you choose to publish them and later receive a request from your subjects to remove them you should do as they request. Even if you wouldn't legally have to remove them it's just the right thing to do.