What the law says
In California, as in most jurisdictions, exceeding any posted speed limit is considered speeding. You can be ticketed for going 1 MPH above, and I've seen it happen. You can also be cited for speeding if you are driving faster than a reasonable speed for the conditions, even if it's slower than the posted speed limit.
It's all about staying "street smart".
Exceeding the posted speed limit is extremely common on California freeways, even in the right lane. In fact, driving at the posted limit slower than the flow of traffic is likely to be unsafe. If you drive no faster than the surrounding traffic, and you stay alert and slow down for the Highway Patrol, you are unlikely to get ticketed. Everyone follows the same reasoning, counting on "safety in numbers".
If you are the first car in a pack or the fastest car on a wide-open road, you are definitely at risk for a speeding citation.
That said, if you do get cited for speeding, "going with the flow of traffic" is a defense that is not likely to work in court.
None of what I said above constitutes legal advice.