Facebook didn't develop their own BB, Playbook, or WebOS/TouchPad apps, either. I doubt HP or Palm were in bed with them or owned ~2% of the company...
Facebook is spam central because of that API, and Twitter is FULL of spammers I had to protect my tweets to stop spammers from Auto-Following me and sending me messages, among other things. Don't get any of that on Google+.
And anyways, a lot of Google+ users (or even Facebook users) aren't really active on the site itself, but on other sites that connect to them. There's a bunch of services that only allow you to sign up via facebook and when they auto post to your wall ofc Facebook says "yay we got another active user" while other users groan at the spam showing up in their news feed.
Until a Social Network has a way to combat the spam, I don't think they should have a R/W API. The benefits aren't worth the risks when you can develop your own high-quality first party apps for high-marketshare platforms.
IMO, Google+ is more similar to Pre-Wave 4 Windows Live than Facebook in how they're implementing the service. There isn't much benefit from it if you're on a different ecosystem. You're better off just sticking with Facebook in that situation.