Just because people know you use X to influence them, it does not mean X has no influence on them. People are not good at properly discounting evidence. Also, some people may not realize you have a system.
If some kind of birthday message becomes too frequent, you can modify your message to stand out of the crowd. For example, send it one day earlier, or one day later. Or use e-mail instead of Facebook; and insert a funny picture.
Just because people know you use X to influence them, it does not mean X has no influence on them. People are not good at properly discounting evidence. Also, some people may not realize you have a system.
If some kind of birthday message becomes too frequent, you can modify your message to stand out of the crowd. For example, send it one day earlier, or one day later. Or use e-mail instead of Facebook; and insert a funny picture.