Things like a standard WordPress/Geocities blog or RSS feed do not have an address book or shared group calendar.

Also, think about the "record locator" for a WordPress site or RSS: it's a _url_. In contrast, the "record locator" for Facebook friends is a [person's real name] or [phone number] or [email address].

It should be obvious that the ramifications of those differences are profound.

E.g. a new user signs up for Facebook, then uploads her contact listing, and then instantly sees suggested contacts including family members, coworkers, old high school & college buddies.

Yes, the sign up steps for Facebook is easier than WordPress but beyond that, the social dynamics of discovering other accounts and sharing are more optimized for the typical ~2 billion users.

Today, plenty of blogs still offer RSS feeds, but without Google Reader around to easily allow people to subscribe to them, they just aren’t being used as much. Also, because nowadays blog owners feel such pressure to monetize, RSS feeds tend to be hidden and discouraged because they could allow users to read content without viewing advertisements. I know one cranky and avaricious blog owner who will automatically ban you from his site if you point out that the site has an RSS feed.