Many CEO’s and CFO’s look at outsourcing anything that is ongoing as a bad thing. If someone is going to pay a guy to work 40 hours a week on a task it might as well be your company, rather than a company that is going to mark that guy up. This would be true, if you only needed A GUY, but in many cases you need tools, software, and relationships which might not come with an employee.
In most cases you will want to outsource SEO, make a decision on SEM, and insource most of your social media interaction. SEO is the hardest of the three to insource. SEO is not just a skill set it is a collection of tools. Much like your doctor is a guy, and he can do a lot when he is in your home to treat small things, it is unlikely that he will diagnose your cancer at your bedside.
A good SEO brings tools like a Google Search Engine Appliance for assessing page quality scores; analytics software for tracking not only your site but your competitors; relationships with hundreds if not thousands of sites; account reps at search engines to expedite questions problems and complaints; and monitoring tools to give early detection to problems with rank, reputation or uptime. A good SEO is also benefitting from working with multiple sites so they can see trends across sites.
These are difficult to insource. A Google Search Appliance approaches $100,000 the first year, competitive analytics software is expensive to build and require constant tweaks to maintain accuracy. Working with a smaller set of sites changes in search engine behavior may be detected too late to make changes to prevent lower rankings. SEO has almost no insourcing advantages. The few that you have are that by having an onsite SEO you can include them in more decisions and embed SEO in the corporate culture. Even this can be done with an outsourced SEO who benefits from the tools of their company.
SEM can go either way with equal success and is often more a function of who is going to have more buying power and how many hours a week do you need. SEMs success or failure is most dependent on the copy used, the landing pages created, and the cost of the ads. Insourced or outsource the first two will likely be the same if you hire the right person. The cost of the ads through Google and Microsoft are likely going to be the same unless your ad budget exceeds $500k a year. If your SEM brings buying power to the table, either through bulk, or through relationships with websites where your ads could run, then outsourcing makes sense. If you hire the right person who can bring those relationships, then insourcing makes sense.
NEVER Trust interacting with your customers to a contractor. For this reason you should insource at least that portion of your social media. Event planning, site building, social network infrastructure planning, and seeding, however you can in or outsource. Likely there will be portions of this that you need help with from time to time which makes it worthwhile to partially outsource this. Even as a consultant I routinely subcontract to event planners for many of the logistics of my social events. Building a FaceBook Apps, integrating with twitter, are all social media marketing tasks, that are likely not aligned with your core business, and make a lot of sense to outsource.