Any major platform has import tools from the other large platforms.
In fact, it's ironic that Posterous once wooed users by building simple import tools to pull in data from Wordpress (among others). Now, people are using Wordpress's import tools to pull their Posterous blogs back into the WP platform.
If you have your own domain name set up, rather than a free subdomain tied to the blogging platform, there is no issue of worrying about people finding you. Same goes for using RSS if you use Feedburner or have your feed available at a standard location on your own domain.
In fact, it's ironic that Posterous once wooed users by building simple import tools to pull in data from Wordpress (among others). Now, people are using Wordpress's import tools to pull their Posterous blogs back into the WP platform.
If you have your own domain name set up, rather than a free subdomain tied to the blogging platform, there is no issue of worrying about people finding you. Same goes for using RSS if you use Feedburner or have your feed available at a standard location on your own domain.