There is one way to get people to use something you build: Make it part of their daily routine. You can do this in two ways:
Applications in category one might include Facebook, GitHub, Gmail, and Microsoft Office—things so invaluable to the people who use them every day that they were willing to adjust their daily routine and fit the apps into it.
But most apps or activities on my computer don't fit into my routine. Uploading screenshots doesn't. I do not want to take a screenshot, open Transmit, and put it on Amazon S3 or some other server to share with friends.
Cloud App solved that problem. When I take a screenshot it automatically uploads it to Cloud App and copies the URL to my clipboard so that I can paste it into Campfire, IRC, or an IM to a friend. I hardly realize I'm using it and that's exactly how I want it to be.
At GitHub we have a lot of internal tools. A lot. So many that we made an internal tool that keeps track of our internal tools. A short list might include things for keeping track of how much money we make, tracking front-end performance, tracking back-end performance, posting status messages about current projects we're working on, and more.
No one wants to remember a bunch of URLs or cycle through a favorites list to post a screenshot of what they're currently working on, so we built an internal app that lets us post screenshots and status messages easily.
We also try to build everything into Hubot, our Campfire bot. We're all sitting in Campfire all day, it's already open, and we can type short commands to get information to help us with our work. If someone wants a graph of our job queues over the past few minutes to help diagnose a bug, all they need to do is type hubot graph me -15mins github.resque.queues and it spits out a graph into the chatroom for everyone to see.
Even if you're application is something people use every single day, you should make the parts that are tedious easier. Google made marking things as spam easy by automatically finding spam really, really well. They made the worst part of email something I hardly even think about anymore.
Do that. Make the tedious parts of what you build easy. Make things that fit into what I am already doing on my computer all day.