What is the greatest challenges facing marketers in the digital online space?

Understanding and interpreting the data since it is the main element of keeping the communication open with the customers.

I agree with Ghady, Cynthia and Reem that interpreting the data is key.
However, the challenge remains in keeping the momentum going, as well as maintaining creativity at its peak and being ahead of the game at all times.

Changing technologies and platforms...
New ways to interact introduces new metrices which, even though allows you to measure further and more effectively, makes overall reports more complex.

I would say understanding, measuring and interpreting data and staying up to date with technology and the latest tools.

I would say acquiring accurate demographics and insights based on a region or a specific country.
There has been times were we got insights that were no where near accurate! On social media platforms, it is difficult to target countries like Saudi Arabia when you cannot see trending topics in Saudi! That's another challenge!

One of the biggest downfalls I've come across is a lack of understanding on a very basic level, across an entire organization or company.
Digital marketing moves quickly, too quickly for many key decision makers.
Security issues, social media optimization, search ...
it's beyond the scope of knowledge for many.

Greatest challenge I am facing right now is syncing with clients.
There is a lack of awareness how things work in Digital World while clients wants to see results immediately.
I have worked for dozens of clients and they won't understand the importance of Website audit, Analytic analysis, and gradual improvements.
Hope to see some better understanding prevails among the general public so they understand what challenges a Digital Marketer might face when plan a strategy.

As a marketer, I always worry about changing customer needs.
Real-time is what truly defines the digital era, and with real-time comes the challenge of dealing with rapidly changing customer tastes and needs.
Another challenge for a digital marketer would be to effectively define his target market.
Market segments as we know them are increasingly blurred and could become difficult to set apart.
Understanding and interpreting data, engaging audiences, and keeping up with latest technologies are other problems that marketers might face in the digital space.

is one of the easier interview questions you'll be asked.
When you are asked questions about your strengths, it's important to discuss attributes that will qualify you for the job.
The best way to respond is to describe the skills and experience that directly correlate with the job you are applying for.

Consumers changing lifestyles and understanding of digital environments are now a constant reminder that companies need to change their marketing approach or fall behind.
With Social Media now at the forefront of young consumers, how many companies fail to understand the impact of this and fail to communicate using this approach? Also, with tablet devices at an ever increasing trend, how many websites are optimised for tablet viewing/interaction? The list of challenges for companies in growing as the demand for new technology increases.
Overcoming or aiming to adapt to those challenges is the solution and companies need to understand that success is simply down to interaction - positive interaction yields results, negative interaction yields pain.
List to your customer and they will lead you on the path to success - ignore them at your peril.

I do agree with Cynthia Sfeir