If you are someone who uses chat programs such as AIM, ICQ, Yahoo messenger, MSN, or Googletalk and have a lot of online friends, it can be a great way to get people to read your blog and to leave comments on it.
As it is with your signature on your email, you don't want to just send an instant message to people everytime you update your blog. You also don't want to simply send a message to someone asking to read your blog.
For someone who has been on your friends list for a while, you can be less formal of course, but you want to be polite when you talk to people who you may have not talked to in a while or someone who is new.
Use your own judgement when you are chatting with people about your blog. If it's someone that you haven't talked to in a while, and someone asks you what you have been up to, you can point them to your blog so that they know what is going on with you.
Some blogs, like LiveJournal, have their own messaging program, so that's also another good way to advertise your blog.
Be certain that you don't send someone a message though only to advertise your blog. It's more polite to say something about their blog first, and then tell them about your blog.
If you have a common interest, it's a good ice breaker and it will give you something to talk about.