The answer is not as easy as it seems. It depends on where your visitor is on his or her customer journey, whether they get what they want when they visit, and how your web pages line up with their goals.
The online visitor and the Brand Loop journey
First of all, forget the sales funnel—in today's digitally mediated world, we have the brand loop. Your visitors are at somewhere on this journey.
Prospective visitors at this stage are just learning that you exist. They're likely looking for something that you offer and have googled it and had your page come up in their search engine results.
Visitors at this stage are aware that you exist, but haven't made a decision to choose you yet. They're learning more, exploring, and comparing. They're evaluating you for fit.
A conversion is when a visitor performs an action that you desire. This could be purchasing something, calling for an estimate, subscribing to your newsletter, donating, or any number of other desired actions.
A loyal customer is enjoying your brand, bonding with it, and advocating for it. Loyal customers are awesome. Sometimes, however, something happens, which brings us to ...
Sometimes customers leave. Just like relationship break-ups, they could be messy. A customer who has left the inside loyalty loop may have done so because you did something wrong, or because he no longer requires what you offer at this time. Maybe they were simply wooed away by the competition. At any rate, they still might visit your site.
The online experience of your website
Thanks to search engines, chances are when they get to your site, they're not coming in through the front door—especially if your website is working well. A well coded, organized, mobile-friendly website with optimized content will get 80% of its traffic to pages other than the home page.
The key there is well coded (responsive design, fast-loading, robust build and accessible interface). If your site isn't usable, you'll lose the visitor. At the very least, you'll drain the visitor's reservoir of goodwill (that goes to a PDF, by the way), which makes it much more work to get them to do what you want them to.
What do you want visitors to do?
Here's a hint: What do they want to do? Websites need to meet user goals, not just yours. So, write and design your web pages to help them along the brand loop journey.
Congratulations! A customer who searched for what you do or offer and clicked on a link to your site is now (almost) aware of you! Help them by ensuring each page on your site helps them understand exactly what you do and who you are. Here are some examples (shamelessly plucked from my Facebook feed):
Hamna's Artistry—their Facebook page adds the tag "makeup and hair" to the page name which is great. However, they should also do that on their website, because what they do is not clear from their name, especially if you enter on a page other than the home page. Better yet, they should add the super-short description on their Facebook About tab, to their header under the logo.
But hey, at least they have a header on all their pages. And their name—unlike this site.
Indicators that your site is meeting the needs of customers at this stage are referrer stats, new visitors, and keywords/Adwords results.
Remember that visitors at this stage are considering turning into customers. Help them by meeting their needs. Make sure every page on your website offers related content and links and calls-to-action to learn more. Have testimonials—not on one useless "testimonials" page, but threaded throughout the website. Here's a great example of this at work.
Your goal at this stage can be measured by increased time on site, low bounce rates, and returning visitors or direct traffic.
The moment of truth: will the visitor do what you want her to do? Well, you won't know that unless you know what you're asking. This is very straightforward if your measure of conversion is an online shopping cart transaction, but for public relations campaigns, this might be a little foggier. Let's look at some examples.
"Try us out"—Buffer is great at this, and nice about it, too. They use a tool called the "Hello Bar" to not only keep their call-to-action across the top of the page, but also to keep it out of the way of you reading the content on their blog. Then, after you've read most of the blog post, they have a slide-up tab that invites you to try the tool again. The message is, "We hope you found that blog post useful. We didn't want to get in your way till you were almost through; now that you are, we'd like to continue to be useful: try our tool It's free!" Here's a great post on their blog that demonstrates this.
"Donate"—Doctors without Borders has a "donate" tab in its top navigation, meaning that it's available on every page.
"Close this window and never come back"—This isn't really what sites like Design Taxi are after, but it's certainly implied with interstitial ads, pop-ups, and other obtrusive calls-to-action. Try this link and see what I mean. Remember, conversion is what happens after a visitor gets what they need from you, how they want it. Don't get in your own way by getting in theirs.
Measuring conversion success is easy: track subscribers, click-throughs, donations, and other transactions.
Returning customers who keep using you or saying nice things about you are the greatest. Your web pages can help them do that:
Give people a reason to return—No one likes seeing the same stuff every time they return, so make sure you're adding new content on a regular basis.
Help them talk about you—have direct sharing buttons to social media.
Measure loyalty with social shares, returning visitors, and social referrals.
Yes, people who are no longer loyal to you may return to their website. They're certainly going to come when the break is fresh if you caused it with bad customer service or products, so make it easy for them to complain at you, not at everyone else. Have useful contact information on every page: a phone number, email, or link to a form. Have it at the top and the bottom, especially for mobile users.
For older breaks, treat these people like those in the consideration phase. You won't avoid those just there to get fodder to rant about you elsewhere, but you may win back those who left for your competitors.
What converts online visitors into customers? Sites that make it easy to go from visitor to customer, take your users' needs into consideration, and do the job they have to do.