I see that some blogs have 10,000 or even 100,000 ...

I believe the obvious answer is that they're doing a very good job... but to get a broader view, such as how many blogs have that many subscribers and what differentiates them from low traffic blogs, I suggest that you go over the recently published Technorati: State of the Blogosphere / 2008 post.

Most of these guys have been around for a long time and have attracted a large audience with good writing.  If you are good at it, people will listen.

Take care,
WebhostAnswers.net

Most of these guys have been around for a while and have been writing constantly about very popular topics. To get a clue what people are searching for you might want to check out technorati´s top search terms or other keyword rankings. And they probably have been posting a lot in forums and discussion groups, thereby amassing a huge amount of back links. Back links are one of the most important factors in getting a high page rank, so your blog will show on one of the first pages of the search results.

Social Media Buttons help your visitors share the content of your blog or website with others. Not all of your visitors will use them, but a large percentage of them will, and this can potentially generate a significant amount of traffic to your site, not to mention another backlink to boot. No savvy internet entrepreneur is complete without a decent set of social media buttons for their blog, although be cautious when adding them, so as not to add too may as it may clutter your site. Whatever your style, there is an easily implementable set of buttons out there for you to include on your blog or website, with little or no fuss at all. We have found the perfect one’s for you right here.

6 ways to share Social Media

Good content!

Also try this Twitter idea:

The number one way is to write great content with iinformation that people will find useful.  Would'nt you subscribe to a feed is that blog has useful information that helped you again and again. Its hard work, but the rewards are great.

As to the other answers you have are very good plus here is my position in aquiring more traffic . plus follow below an check out a guide to increasing your blog traffic or here > http://moretipsonacceleratingyourblogtraffic.blogspot.com/ plus here >  http://betterbloggingguide.blogspot.com/  plus here > http://aguideforincreasingyourblogtraffic.blogspot.com/  plus Go to my page by clicking on the picture  . you will see not only to the right but below my picture a link to one of my blogs , I have 60 + blogs on various subjects . Another note as I see you haven't participated in answering to many others questions but it is a really good way of refering an attracting traffic to your blog it even might give you a little inspiration as to creating other blogs. I even put some of my answers here in my blogs  IE The ones about painting tips> http://paintingtips-dirtdog.blogspot.com/    another is about bipolar realities etc >  http://variablesinbipolardepression.blogspot.com/ all are created on my blog first an then copied an along with the link I say if you would like to see this answer along with additional tips an answers go here is my profile link please check it out to see all of my blogs > http://www.blogger.com/profile/17420780266615900056 There you will find additional tips for accelerating your blog traffic etc. there is several blogs there that may be of interest to you . I certainly hope this helps in your work feel free to send me a message with your blog URL address and I will be more than happy to take a look at it . maybe throw some traffic your way. Also you can put your yedda answers on your blog see any of my blogs look to the right for my yedda link profile  to reverse traffic back to here all links give your ratings a better standing in the search engine optimazation ranks .

Cheers                                       Dirtdog

There are two basic things that leads to getting such a lot of subscribers :-

1. Unique, informative and interesting blog with frequently updated content,

2. Giving away of an useful free ebook or something else to subscribe,

3. Age of the blog may also sometimes be a factor.

For more information on blogging and related matter you may read these two posts -

1. How to start a blog

2. How to start a blog to make money

Hope this helps you.