Tumblr has that follow and stumbling upon features which might aid discoverability of your startup company blog, but it seems to be very SEO unfriendly. I have not used posterous. And self-hosted WP you can SEO to your heart's content. I'm curious as to which some of your startups chose and why.
Tumblr / Posterous is useful for quick, occassional content if you don't have the time or skills to optimize Wordpress.
Otherwise, the ability to EASILY integrate analytics, feedburner, other content, SEO, change things at will and plugins til your heart is content mean I don't really consider the other options!
Purely from a SEO perspective, self hosted Wordpress is way ahead. With specialized themes like Thesis, you can remarkably increase the SEO and make it easy to manage.
Tumblr is a very good option for a personal blog or for the blog of a company which already has good traffic on it's website.
We have used both, and I can assure you, you would feel very lonely if you are on Tumblr and looking to generate traffic through your blog.
In short, if you are a startup looking to generate traffic, and hopefully sales from your blog content, then there is no better option than self hosted WP with a SEO friendly theme.
I'm not sure that my company is really a "startup" anymore (I started the company in 2003) but one of the best web decisions we made was to use self-hosted WordPress for our website. There are many, many users creating a huge array of plugins and tools for it, and it makes posting and SEO work very easy. If your startup has tech-savvy people in it, or if you can afford paying someone to help customize it to your purposes, you will do well with it. If you'd like to see our site, go to www dot trustworthycare dot com.
Tumblr is more of a personal use which makes things quick in terms of publishing the content, etc. but sucks at SEO. If you are serious about getting leads from blogs, I think it's best to do a self-hosted WP, you can do pretty much anything!
Don't forget that you can also use the hosted version of Wordpress, but you'll need to spend some money for Ads not to appear, to have your own domain, and for a few other nice to have things.
Then there are also services like Squarespace which advertises on podcasts around the place.