When two people are introduced, it is generally by a mutual friend or family member. Many times they are the people who have been searching hardest to find matches for friends and loved ones, and they want all the details when they set two people up for a night out. It might seem like a good idea to tell them exactly what happened, but keeping them out of the relationship from the beginning is important for its eventual success.
It could seem rude to tell someone who has taken the time to set up a date that is it none of their business what happened, but there are polite ways to say it. Giving them a positive or negative answer should be enough, but they will often press for details. It is best to simply tell them if it went well or badly, and then it is time to change the subject. If the dater does this every time their loved one brings it up, it will eventually give them the hint that no more details will be forthcoming.
The reason it is best to start off this way is to let the person know they will not be part of the relationship, and the interaction between the couple is private. Telling them everything right after the date gives them the feeling they are in control, and they further believe they have a right to know intimate details. It sets the standard for a lack of privacy between the two people, and it gives permission for an outside person to be part of the relationship.
While it is not always easy to do this, it is best if the relationship is to continue. Keeping personal details private sets the standard for healthy boundaries, and they will continue as long as the relationship lasts.