A mob corresponds to an exact web address, or URL. This is an important point for mob creators to understand, especially if they're promoting their own site. Every character in a mob URL matters: the address, http://www.mysite.com results in a different mob than http://mysite.com. While entering either in a browser address bar may bring you to the same place, the two addresses represent separate and distinct YouMob mobs.
This is why we encourage mob creators to copy the URL from the browser address bar after the web page is displayed for the site they want to mob. When that URL is pasted into the YouMob Start a Mob page, you're guaranteed that the address matches exactly the address that the site/page actually resolves to.
Even better, we encourage mob creators to use the Start a Mob quick-start button that can be added to your browser favorites list or link bar. This will automate the process of getting the mob URL just right.
Also note that if you're using a service like badged.net for placing bookmarking site badges on your content page, you want to make sure to get your destination URL just right. Make sure the URL you enter for the badge matches up with what the browser address bar shows when visiting your page.
One last comment regarding building a mob around your blog. You probably want to have a mob for your blog site (http://youmob.blogspot.com, for example), rather than for individual blog entries. While the latter is possible and even useful, remember that you may be competing against yourself for Most Mobbers placement by having some of your traffic visiting specific blog entries as opposed to having everyone mobbing around your blog entry page. It's up to you, but something to think about.