Have people in California been contacting their Senators? Looks like it might have helped as Senator Feinstein joined as a cosponsor earlier this week:
Details for S. 395: A bill to amend the Internal Revenue Code of 1986 to extend certain tax ... - GovTrack.us
Inspired by your post, I talked to someone at one of the original cosponsoring Senator's office, and at least their office treats opinion/feedback online and in person equally, and confirmed what you've already done with friends -- getting more people to reach out is indeed the right way for Congress to realize it's important especially for legislators that typically vote a different way and might need to see more evidence of support.