Depends what, why and where.

If someone recommended it to me and the service was not good, I let them know.

If I have very poor service, I ensure that people thinking of using that service know. I also will put in a complaint if needed.

If the service is bland, I say nothing but generally don't return or recommend. I like good service, but mediocre service isn't something to complain about.

Example: shop where assistants (young and unsupervised usually) are more concerned about their night out than service and so do not do a good job. I stay away and would not recommend. Meal out where it's a little too full with too few waiters, then I don't put it on my list of places to return, but would try again if others wished to go there. Health "care" we've had in hospitals in the last 10 years... that's very poor and people do know about that! Although they do nothing about my complaints.