I see some companies spend a great deal of money marketing their business online only to shoot themselves in the foot for missing out on some very important steps that can really help out the marketing and link building for a website.
Here are some of those SEM related mistakes:
Forgetting Links: I couldn’t tell you how many times I have been to a business fan page on Facebook or a Twitter profile where the field for a link to a website is blank. Seriously? That is one of the most important pieces of information you can leave behind besides your logo. A link should be capitalized on when or where ever possible. Even in YouTube descriptions there should always be a link visible for others to use to channel over to your website.
Lack of Conversion Metrics: Why would you ever want to spend money sending people to your website when the site itself has not been scrubbed with a fine tooth comb for errors in conversion metrics? This should be the first starting point.

Skimpy Onsite Optimization: Did you take the necessary time to optimize your website before you started all that hard work on link building? I hope you used a tool to conduct keyword research and not just using your head to come up with keywords.
Forget A Logo: If you can’t take the time to upload a logo into your social or business profiles than you deserve to fail. How is anyone ever supposed to make a connection with your online brand if they can’t recognize a logo? No joke, I actually came across a few decent sized companies recently on Twitter that still had the default Twitter image for their logo. That is just mind boggling.
Take the time to think about every step you are doing online from a logical stand point. If you are trying to build up a brand or deliver traffic to a website there are certain measures that need to be visible in order for everything to work how they are supposed to.