You can take different approaches to measuring the success of your online activities.

Traffic-based.

You’ve got high click-through rates on your emails and other marketing channels, so you’re generating a lot of traffic. You’re essentially interested in achieving scale, measured by Unique Visitors, Visits, or Pageviews. Perhaps, bounce rate is considered and you want it to be low, with more pages per visit viewed.

This approach is fine for an ad-based website that is getting compensated for pageviews. If you’ve ever been to a top 10 or 20 list, you’ll have seen this approach in action, when you’re required to browse to another page for each list item.

Basic Engagement.

However, if your website is intended to engage users to act or become more informed about a topic, you may want to consider measuring the quality of the traffic coming to your site, not just traffic numbers.

Some ways to measure engagement:

time on page/site (visitors are reading the article) -
OK but number of visitors that spent a minimum amount of time on your site is even better
low bounce rate (visitors are interested in related content)
OK but be careful. You can have a low bounce rate if most visitors hit pages looking for something of interest but don’t find it. Take a look at what single page visitors are doing.
downloads (you’re offering an asset of interest to your target audience)
OK but you don’t really know what was done with the download. Did the visitor read that PDF or not?
exit links (you’re helping visitors find a helpful resource outside your site’s domain of expertise).
OK but similar to downloads, you won’t know what the visitor did on the external site. (Note that this metric counts clicks on links that you provided, not visitors leaving your site by closing a tab or entering another URL in the browser, etc.)

Note the caveats for each individual metric. You can better gauge engagement by combining multiple measures into a single engagement score.

Personalization. With an engagement score, you can identify what content (and how it is promoted) leads to higher engagement scores. If your implementation allows it, you can use the personalization features of your CMS to promote content for individuals according to what content they tend to be more engaged with.

Outcome-based

Engagement is good, but it can be misleading. Are visitors spending a lot of time on your site because they find value in the content? Or are they spending a lot of time on your site, because they’re struggling to find what they need or want? It could be that single page visits are delivering more than multi-page visits.

Outcomes, come in to play here. Some outcomes will be discrete:

purchased a product or subscribed to a service
became a member
donated funds to a project.

Other outcomes will be less tangible (particularly with social mission web sites). For example, visitors became more informed about a topic that impacted their lives

With discrete outcomes you can directly analyze web behaviors to understand what behaviors lead to the desired outcome. Less tangible outcomes require a survey or some other mechanism to get direct feedback from visitors and an ability to identify what web behaviors are aligned with their responses. For example, through a survey you can ask visitors if your site helped them to better prepared for their retirement.

If your analytics implementation allows you to group individual responses with overall behavior, you can gain some key insights. What types of website engagement are leading to desired outcomes? What marketing channels lead to desired outcomes? What content/articles/assets are resulting in those desired outcomes?

Optimizing Outcomes. If you can identify what web behaviors and content consumption leads to desired outcomes, you can then develop and promote web content/assets accordingly. For personalization, it’s possible you can identify visitors that are engaging in web behaviors that aligns with a particular outcome and promote content/assets to them accordingly.