The author of the excellent Micro 4/3rds Photography Blog reveals the numbers behind his blog – and with blog ads, commissioned sales links and Youtube videos with ads, the combination earns very little money.
Most of the top video channels on Youtube got started very early back when it is alleged most channel creators gamed the system to increase viewership . While some recent arrivals have achieved viewer success, it may take years to achieve a reasonable following, or money spent on promotion. It is said that even for the successful Youtube channels, the creators most creators must also sell ancillary products (notably music but also t-shirts, hats, etc).
So why blog or produce Youtube videos? For many of us, do these as a hobby. We enjoy sharing helpful information with others. For example, I publish a popular tutorial blog on programming in App Inventor (See App Inventor 2: Learn to Code!). Other reasons include emphasizing one’s credentials in the subject, to advertise one’s skills to potential clients, to be involved in a social web of people with similar interests, perhaps to sell related products and services, and more.
Footnote – The “old” way Youtubers Got Views
 Many Youtubers figured out the way to get viewed – and obtain subscribers – was to generate lots of fake views to jump up high on the “Most Viewed” lists. In the early years of Youtube, it was easy to do this: copy the video player embed code 100 times on an HTML page and just keep reloading the page! (This “feature” was disabled long ago!)
Appearing on the “Most viewed” lists increased visibility and views rapidly. Others say that nearly all of today’s successful channels, which started in the 2005-2008 first three years of Youtube, got there by manipulating view counts.
In 2012 or 2013 Youtube seems to have made changes in the search system that caused viewership of minor channels to fall. Starting about 2 years ago, views of my own videos fell sharply – and I no longer post much on Youtube, having instead migrated to Flickr (Flickr supports videos too!) On Flickr I often get as many views in a day as I would see – ever – on Youtube 🙂