Yes, downsizing can make noise look lower because neighboring pixels are averaged together, which smooths fine-grained noise. But at the same final viewing or print size, the benefit is usually modest: you’re mostly trading finer noise for smoother/coarser noise rather than gaining a dramatic low-light improvement.

Best practice is to shoot at the camera’s full resolution and downscale later if needed. That keeps all your detail for future cropping or larger prints, and you can always reduce size afterward—but you can’t recover detail if you capture only a smaller file in the first place.

If your camera offers RAW, shooting RAW at full resolution is even better, since it gives you more flexibility for resizing and noise reduction later. If you shoot JPEG, you can still downsize afterward, but you have a little less latitude.

For online sharing, there’s often little reason to lower resolution in-camera anyway, since sites like Facebook will resize images themselves. If noise is your main concern, dedicated noise-reduction software will usually help more than simply lowering resolution in-camera.