If someone uses your site, app, or service for a year, reward them, celebrate them, thank them. It's one of the simplest ways to improve your relationship with your customers and few companies do it.
I actually can't think of any company I frequent that acknowledges ongoing loyalty, outside of displaying "Member since" on an account page. These are likely your most valuable customers. Why not do everything you can to reward and keep them?
The bar is so low that just a short, thoughtful email from the CEO each year (which could be written, designed, and automated in a day or two) would launch a company into the top 5%.
Thank you for being a customer! You made your first purchase with us one year ago today. We hope we exceeded your expectations then and every time since. If we haven't, please let us know so we can improve. We look forward to working with you for many years to come.
That's it, really. Acknowledge the milestone, show appreciation, and provide an easy way to respond with questions or concerns. Another option is a note inside the site itself. The more personal you can be, the better.
Rewards are an option, too, such as a discount on the next order, free shipping, or maybe early access to a new feature.
Subscriptions are a little different. We often use pricing to encourage loyalty through the early-bird approach; be one of the first to sign up at the temporary, lower rate, then keep that rate as long as you remain a customer. That works well for your very first customers, but what about the many more who sign up later? What if the subscription price actually decreased each year someone renewed, or the benefits increased?
Most companies recognize employees by celebrating anniversaries and rewarding loyalty through increases in salary, perks, and vacation days. We could value our long-time customers, too.
We have all of these fields in our databases for a reason. Find your loyal customers and let they know how much they're appreciated.