I am no germa-phobe, but what about germs? I believe the immune system is boosted by having a not so sterile environment at home, but there is a time when the number of germs on your hands can cause more illnesses than necessary.
My husband's oldest sucked her thumb so much as a toddler that cracked and bled. Her solution? Start sucking her finger. We managed to get her to stop sucking her finger (she never did go back to the thumb) when she was with us, but as soon as she saw her mother, in went the finger.
We finally started talking to her about how sick she could get if she kept sucking her finger (she was sick a lot as a child, in my opinion). Peer pressure had nothing to do with her choice to quit sucking her finger. I would go into her room at night when she stayed with us and pull her fingers out of her mouth if they were in there. She would fall asleep with them there, but once out, they stayed out. She was nearly 9 when she finally quit for good.
Neither of our younger kids sucked their thumbs or fingers. Our son just loved the bottle. Our daughter took a pacifier until she was about 6 months old then spit it out one day. End of oral fixation as a child.
Try the germs approach. You could do that germs experiment where you put glitter on her hand and then have her walk around and touch things, like doorknobs and faucets and toys so she can see where germs can be. Explain that the glitter could be germs and, while some germs aren't going to make you sick, lots of germs certainly can. If each of you do it with a different color, she can see how she can get germs from lots of people. She might get the message.