Keeping track of KPIs is a must for all businesses, and measuring marketing KPIs is a must for all marketers. Use tools like Google Analytics, KISSMetrics or other tools out there to track your KPIs and extract valuable information from this data.

Here are some KPIs you should keep track of:

Overall site traffic: When measuring site traffic, don't only focus on page views but also on how many unique visitors your website get per month or per week.

• Mobile traffic: Increasingly more people are accessing the internet through their smartphones and tablet computers, which gives internet marketers more revenue sources.
• Cost Per Click (CPC): CPC value is determined by how popular the key words you have chosen are, your Quality Scores affected by Click Through Rates (CTR), and also the normal costs search engines initially set.
• Traffic sources: Understanding where your traffic comes from, whether that is from search, referral or social, will help you strategize your digital advertising effort.
• Click Through Rate (CTR): The larger the CTR, the better your Quality Scores will be, allowing you to lower your PPC prices by receiving pricing reductions from Search Engine Marketing platforms like Google Adwords.

Conversion metrics: The primary goal for your digital advertising effort is to convert visitors into clients. These metrics will change depending on your business model.

• Conversion rate ( CVR): Whether your purpose is convert site visits into sales or to assemble valuable information about potential customers and your website visitors, tracking your Conversion Rates can define your digital marketing success.
• Cost Per Lead (CPL): Defines a particular campaign cost over the lead conversion ratio, giving insights on how lucrative the campaign is.
• Bounce Rate : Some visitors immediately leave or "bounce", this metric is important to assess your weaknesses. An interesting way to look at it is to cross it with each traffic sources.
• Average Page Views : The more page views; the higher the chances are that your visitors will engage with your website or product. This can help you determine a good lead from a bad one.
• Average Cost per Page View: Your price per page view ought to be much lower than the revenue you are able to generate from the page.
• Average Time on Site: This is necessary for judging what content is relevant to which audience, and which drives conversions to help you prioritize your content creation.
• Rate Of Returning Visitors: The importance of this metrics and how you use it will depend on your business model. But returning visitors are almost always a good thing.

Revenue Metrics: These metrics are the ones that management is most interested by, and will probably influence your budget for the periods to come.

• Return on investment (ROI): This can help you assess your overall strategy and prioritize on areas that provide a higher ROI, while testing on the ones that have lower ROIs.
• Cost to Acquire a Customer (CAC): Keeping this number as low as possible should be one of your priority. This metric enables you to rank your marketing campaigns on an effectiveness scale.

Here is a great resource if you want to dive deeper in marketing metrics.