There are many ways you can learn. Some are considered formal, usually the ones related to being enrolled in a class. It is expected that these award you with some kind of certification that validates you have acquired certain knowledge or skills. Basically, these are the bullet points that go under "Education" in your resume.
Nevertheless, I have come to realize that I have learned so much more informally than formally in the past years. The reason: Personal Learning Networks (PLN). First, I belong to closed groups of incredibly talented educators, such as the Apple Distinguished Educators and Google Apps for Education Certified Trainers. They always have valuable discussions going on. With the use of Twitter, I see what thousands of people are doing in education. I find incredible technology integration projects going on all the time, all over the world. And the best of all: it's FREE!
Now, how to measure informal learning? How can you demonstrate what you have learned if you don't have a diploma for it? My suggestion: create content and share it. Videos, blogs, wikis, podcasts, etc.
Smart employers will take a look at your contributions. They might even find out about you based on what you are sharing.
What you do you think? Do you have other suggestions on measuring informal learning?