When deciding who to use for your marketing you have a few options:
Internal staff – You may want to try and keep costs low by getting internal staff to do your online marketing or by doing it yourself. This will only work if the person responsible for it has a good knowledge and has the time to carry out the tasks needed. If not, this can be detrimental and if done incorrectly can actually harm your website more than by doing nothing.
Third party person – You may wish to outsource your marketing to another individual who has experience in the area. This is fine, as long as they have the adequate time to do so and they have the knowledge and experience. They will also need to ensure that they are keeping up with the latest advances.
Specialist marketing company – These companies usually have a number of employees whose main job involves working on online marketing on a daily basis. They usually have adequate staff to cover illness / holiday leave and therefore can offer you a reliable and consistent service. They will often charge more than an individual but sometimes the benefits of using a larger organisation outweigh this.