ScrapeBox has an addon specifically for checking expired Web 2.0 called the Vanity Name Checker
http://www.scrapebox.com/vanity-name-checker
Out of the box it works with:
Tumblr.com
Wordpress.com
YouTube.com
BlogSpot.com
LiveJournal.com
Twitter.com
Pinterest.com
Weebly.com
Flickr.com
OverBlog.com
WikiDot.com
For other sites not included in the Vanity Name Checker, yes you could possibly use the
Alive Checker Addon. Just first use something like
http://httpstatus.io/ and enter the URL of a registered profile and a non registered profile so you can find out the response codes to add to the Alive Checkers options. Generally registered profiles will return a 200 response and non registered profiles return a 404 response, or sometimes a 302 redirect.
The other thing which may happen is the Web 2.0 will return the same 200 response for both registered and unregistered profiles so you cant tell them apart by the response headers, but instead they will print a specific text on the screen for example "Invalid user specified".
For platforms like this, just load up the Page Scanner Addon
http://www.scrapebox.com/page-scanner and add the "Invalid User Specified" footprint and scan the pages for it and any profile containing this should be available to register.