I have been offering free images at 123rf since I subscribed as they promise an increase of exposure of your portfolio and the chance to sell a larger version of the image to the same customer.

I am not sure I have gotten much out of it. The free images are downloaded several times but never a larger version seems to be purchased and my sales since I subscribed back last Dec have only gone down.

I would like to hear people's perceptions about donating images there. Does it really help the photographer in any way? I am thinking to stop doing it.

Here many times already started discussions about donations of images. Many thoughts, opinions posted. I think you need to search this forum. Personally, i never witnessed increasing of sales after donation of somehing for free not in photography, not in any other business which proposed donations. This is only your choice, but think about your time, your eyes, your expenses. Donations i think can be justified when you don't expect a cash flow in exchange (a real and low budget charity for example, which can return to you as positively as well as negatively), otherwise it is a myth.

I offered free image a few times on 123rf. They were downloaded a few times, but I never saw any increase in sales, so I stopped offering them for free.

I do donate on 123 and many of the images are later bought at a full size and also related images are purchased. I rotate a whole bunch of ones that never sold there or elsewhere and they are now getting seen and purchased. So for me, it works and works well.
As for not giving anything away, my philosophy is you need to give in order to get. Just be fully aware of what you are giving.

Never give anything for free unless it is for a family member or a charity that you know and respect.
The business of graphic design, illustration and photography has been de-valued enough without us giving our work away for free.