Set your goals. And then reset your goals.
What you are looking to gain from online marketing? Is it increased awareness? Is it increased revenue? You will probably have multiple goals. Keep track of what they are so you can measure your success. When you reach your initial goals, set new goals in order to continually improve.
Wear a little piece of my Art on your feet 
Marketing departments should focus more on “business” metrics and less on “marketing” metrics. Why? A lot of marketing metrics are “leading” indicators, not “lagging.”
Here are my go-to marketing KPIs:
1. Marketing Sourced Net-New Revenue
2. Contribution to Total Net New Revenue
3. Qualified Pipeline Generated
4. Cost of Sales Qualified Opportunities
5. Win Rate
6. Sales Velocity
7. Marketing Customer Acquisition Cost
8. Marketing CAC Payback Period