Hey mate,
as far as I know, you usually ping a site when you have uploaded new content (or changed something around your site). The purpose behind pinging a site is basically making sure that search engine robots find your new updated content quick enough. There are a couple of services which will ping your site for you (pinggoat, ping-o-matic or pingler are my favourite ones), a ping works equally well when submitting the main site or a subpage. Don't overdo it with the pinging though, as you migth come off as spammy if you start submitting the same site or sub-site more than once a day.
Hope this helps, cheers.

As far as I know is that you ping the new content or articles posted in your site or blog so you need to ping the url of the new articles being posted in your blog...