Authors hear the refrain from publishers—“Build your platform! Build your platform!” But building it is only the beginning. After that, it’s important to know how a platform is performing.

Every platform is different (we are all unique individuals), but here are three specific kinds of numbers that can come in handy when evaluating the size, strength, and/or impact of an author’s online platform.

1. How many people visit the author’s site in a month?

This is easy to determine with Google Analytics. The Google Analytics dashboard has a whole world of data underneath surface figures like these:

Look first at the number of people who visit the site in a month’s time—the default view. See how figures relate month-on-month, and year-on-year. Are more visitors coming over time? What’s the percentage growth month-on-month or year-on-year? Ideally, this is a positive indicator that lends strength to the platform.

Other Google metrics that can be important:

What content is most popular on the site. It’s especially useful to know what information is highly ranked by Google’s search engines so you can say things like: “The blog posts on how to bathe your cat are the most highly ranked for anyone searching for ‘cat bath.’” Or, “The site receives more than 3,000 visits per month from people searching for ‘cat bath.’”

How long people stay on the site. Typically, the more compelling the content is, the longer people stay. This is called “site stickiness.” When a site is sticky, visitors are more likely to respond to calls to action, buy things, click on ads, and more.

How people find the site. There is no “right” way, but match-ups matter. For example, when an author claims to have impact on Twitter and Facebook (see #3 below), metrics should probably show that a good portion of the author’s site traffic comes from those sources. If the claim is visibility though search engines, metrics should indicate meaningful search engine traffic. If the site is “highly recommended by authorities” in a relevant community, figures for referral traffic need to show that.

2. How many readers can the author reach directly via email?

Add the following numbers:

• the number of people who subscribe to the author’s site or blog via email

• the number of people who subscribe to the author’s unique e-newsletter

• the number of people the author would feel comfortable emailing personally about a book or

product launch

The total is the number of people the author can reach directly via email, and it’s a number that’s powerful. Those who don’t currently have a way of capturing email addresses should consider starting an e-newsletter or offering an email-based subscription to their blogs.

3. What level of engagement does the author have through online channels? (Or: What is the author’s ability to get people to act?)

One popular way to determine engagement is to look at Klout statistics (see klout.com/corp/kscore). Klout measures social influence, and once supplied with information about services used, it will start to tally how responsive people are.

Here’s an example:

In this scenario, since I have roughly 140,000 followers on Twitter, and about 14,000 retweets/mentions, that reflects about 10 percent engagement. (Note: Don’t get hung up on the Klout score itself; instead, study how well engaged followers are.)

Other ways to gauge impact and responsiveness include:

HootSuite analysis. Here’s an illustrative excerpt:

Using HootSuite to send tweets provides access to weekly analytics of how the tweets “performed” in terms of clicks, replies, RTs, and so on. So even an author with a modest following of, say, 1,000 people might be able to report engaging 30 to 50 percent of followers in a single day of tweeting. Actually having followers’ attention is more impressive than having a ton of followers.

HootSuite also allows people to subscribe to public updates on Facebook (via a personal profile), which provides another number that helps indicate visibility and impact through social networks.

Of course, an author may have a separate “business” profile page for Facebook; in that case, the goal is tracking engagement there. But for those who don’t have a business page, opening the profile up to subscribers is a great alternative (see facebook.com/about/subscribe).

AddThis analytics.

I use the AddThis plugin for my site because it publicly tallies the number of times an article is shared. This is very useful data to have when making a statement about how well specific content spreads.

Google Analytics can also help track how much traffic comes from social networks where an author is active. Authors who don’t already have it installed on their sites would be wise to install it today.

You and your authors can collect many more metrics to measure the power of an online platform, but these will get you off to a strong start.

Jane Friedman is an assistant professor of e-media at the University of Cincinnati and former publisher of Writer’s Digest. Her expertise on technology and publishing has been featured on NPR, PBS, and Publishers Weekly, and she has consulted with a range of nonprofits, including the National Endowment for the Arts and the Creative Work Fund. She blogs on how writers can empower their careers through new media at JaneFriedman.com.