Please keep in mind that Compete's data is from ISP data, so any http request to plus.google.com will count as a visitor. Which is why sites like, atdmt.com [1], have more "visitors" than google plus. I wouldn't trust compete's data for Google vs Linkedin because, the "uniques" could be coming from integrations with other google properties.

[1] http://siteanalytics.compete.com/atdmt.com/