Google doesn't make it easy to crosspost from other networks to Google+, but that's OK -- turn it around and use Google+ as the feed for your other social networks instead! Here's how to funnel all of your Google+ posts (or just the posts you want) over to Facebook or Twitter using IFTTT.
This tip comes to us from Magenta River Photography, who uses a similar approach to share photos across networks easily. You'll need two tools to make this work. IFTTT, of course, and Yahoo! Pipes, which will give you an RSS feed of your public (not your private or circle-only) Google+ posts. Here's how to set it all up:
(1) First, you'll need your Google+ ID number. Log in to Google+ and click "Profile" on the left sidebar.
(2) Grab the string of numbers in your profile URL (should be google.com/u/0/XXXXXXXX/posts)
(3) Visit this Google+ RSS Link Yahoo Pipe and paste in your profile ID Number.
(4) Run the pipe, then right-click "Get RSS" to copy the RSS link to your clipboard.
(5) Log in to IFTTT and activate the Facebook and Twitter channels (if they're not already). You'll have to log in to each service and authorise them to post on your behalf.
(6) Use this G+ to FB recipe or this G+ to Twitter recipe if you want to post from Google+ to Facebook or Twitter based on a specific keyword. Paste in your RSS URL in the "Feed URL" and the keyword you want IFTTT to look for in order to crosspost in the "Keyword or simple phrase" field.
(7) Use this G+ to FB reicpe or this G+ to Twitter recipe if you want to everything you post to Google+ to go to Facebook or Twitter, with no exceptions.
(8) Activate your selected recipe, and you're finished. Any future posts (or future posts that match your keyword) will be automatically posted over at Facebook or Twitter (or both).