It doesn't work as well on FB. Twitter gets more respect on the business front for some reason. Possibly, because established media often quote Twitter quotes giving them more validity than FB. Selling photography via the web can simply be a numbers game. However, you need almighty, huge numbers to get there. The best way to sell yourself as a photographer is to meet people who may need to use your services and leave a hard copy portfolio with them for a week, or so. Banging on doors is still the best option.