There was a 2014 case that was thrown out in appellate court where the driver was stopped at a light and was using his phone's GPS device. There is functionally no difference between using a GPS app and using the facebook app.

Facebook at a light that turns green and 30 seconds later they are still sitting there.

Then you cite them for obstructing traffic.

Edit: If you read the ruling you'll see that it also would apply to non-GPS applications, I gotta find it though, standby.

Edit 2: Found it

https://s3.amazonaws.com/s3.documentcloud.org/documents/1031494/spriggs-opinion.pdf

While the statute may be interpreted, on its face, as the People assert, we agree with Spriggs that the statute is reasonably construed as only prohibiting engaging in a conversation on a wireless telephone while driving and holding the telephone in one’s hand. This is because the statute specifically states the telephone must be used in a manner that allows for “hands-free listening and talking.” (§ 23123(a).) It does not state that it must be used in a manner that allows for hands-free looking, hands-free operation or hands-free use, or for anything other than listening and talking. Had the Legislature intended to prohibit drivers from holding the telephone and using it for all purposes, it would not have limited the telephone’s required design and configuration to “hands-free listening and talking,” but would have used broader language, such as “hands-free operation” or “hands-free use.” To interpret section 23123(a) as applying to any use of a wireless telephone renders the “listening and talking” element nonsensical, as not all uses of a wireless telephone involve listening and talking, including looking at a map application