Web marketing tends to be very time consuming, especially when you consider how vast it is. Pulling together all the components of SEO, content, website architecture, analyitcs and social media, you can be looking at 5-20 hours per week. The larger the site the higher that number goes. Social media itself can be particularly time consuming to do right, since that involves not just content generation but spending time engaging online with your potential audience and online thought leaders. I would say, however, that on a skill level, that's the easiest to achieve, if one has the knowledge that is required. But every area of web marketing requires a good deal of knowledge and a great deal of skill to ensure success.
I would say the most time consuming part of running business for me is probably making sure everyone is on task and doing their job to the best of their capabilities. If you have a sales team like I have, "babysitting" come to mind. Also, if you are in a tech startup, raising money would most likely be the biggest time suck.
People. Definitely people and making sure everybody is on the same page, whether clients / customers, employees, contractors, vendors, or family.
Your people, their actions and their brains!!
How I handled it?
You must be religious about this: take 1 hour of your daily work and make it a habit to train your employees on a certain subject of your business. Continuously sit and talk to about your services, your products, customer interaction etc... They must understand your logic and reasoning to start doing the same work as you do