Many startups undervalue the importance of knowing their customer/user half life. It's vital to know the relationship between user acquisition and churn and I can not reccomend enough setting up a daily reporting system (even a plain text email from a crontab script) that gives you a sense of your startups heartbeat against historicals (prev day, 2 weeks, 3 months, 1 yr).
Also, a lot of startups develop these wonderful signup systems and acquire user emails but fail to realize the potential of ambient messaging. Even if they don't read the email, it keeps your users thinking about you. RescueTime does an excellent job with their email marketing. Not only is it pertinent, but I find myself going back to RescueTime more often because of my level of awareness for their product. The lack of user retention initiatives in my first startup was likely the ultimate driver of its inability to build community despite a very strong launch (it was still acquired for its tech assets/hr). Now, working on strong online properties, I can see the impact of reaching out to your users. Dollar for dollar, I think email has the highest roi. Much more so than your run of the mill facebook app.
"The most significant finding from our usability research on email newsletters is that users have highly emotional reactions to them. This is in strong contrast to research on website usability, where users are usually much more oriented toward functionality. Even a website that users visit daily seems to feel like a tool: users want to get in and get out as quickly as possible rather than “connect” with the site. Users tend to glance at websites when they need to accomplish something or to find the answer to a specific question. In contrast, newsletters feel personal because they arrive in users’ inboxes, and users have an ongoing relationship with them. Newsletters also have a social aspect, as users often forward them to colleagues and friends. The positive aspect of this emotional relationship is that newsletters can create much more of a bond between users and a company than a website can." - http://www.nngroup.com/reports/newsletters/summary.html