What defines Digital Marketing today and what are the top 3 metrics to measure effectiveness?

The most 3 Important metrics to measure effectiveness are - Average Time of the Visitors on each page - Bounce Rate - A/B Testing and Goal Conversion rate

There are many metrics that can be used for digital marketing but the ones I find important are: Average Time on Site Bounce Rate Average Cost Per Page View

Most important metric i believe would be 'return visits'.
If this is high your low 'avg .time on site' and high 'bounce rate' does not matter.
These could infact mean that your users are getting what they need from home page it self, which is really good.
Second would be interactions: registrations, downloads, etc.
Third would be total visits as this shows how reachable your online space is, be it through good SEO or paid media.

Since your Question is generic the answer is too...
for me the Top3 metrics would be ROI, ROI and ROI.
That being said and understanding the you'd have set goals / objectives to any Online marketing activity the Top 3 metrics would depend squarely on your goals.
However again going generally for all campaigns it would be: Response, Recall & Referrals.
If you got the desired action goal, it would e a success, if you get Residue or Recall, it speaks well for the relevance and cut-thru of your activity & finally if you referrals, you it means your audience cared enough to recommend you to friends etc...
which means they've really bought into you / your message / service.
All this will lead to better ROI for sure.

It depends on the business you own or manage.
Usually, in my opinion I would say the 3 main metrics to measure effectiveness are: 1.
Conversion rates (After identifying "Success" vs "Fail") 2.
Registrations, Signing up for emailers...etc.
(Basically email retrieval) 3.
Average time on Site or Downloads But again, it depends on the business you own or managing.
Metrics are never fixed.

The digital marketing is making marketing using the most popular ways that is most usefull this days by everybody around the word suh as :e.mails,cellular phones,social networks...etc.
There is 5 metrics of Digital markiting: Targets Prospects Leads Opportunities Closed deals

In my opinion, the key metric to measure is cost per acquisition (CPA).
Your job as a digital marketer is to get more customers at the lowest cost possible.
So start by measuring your lead sources and see which source is giving you the best CPA.
Once thats established, scale it and optimise to improve it further.
Everything and every other metric follows.

For me it's: 1.
clickthrough 2.
conversion 3.
customer satisfaction The three 'C's' as I know it....but there are many out there - it all depends on what your objective is.