I know that BBC has decided against a third season, and nothing a viewer in the USA says will likely change that, but here goes anyway: My wife and I watch a lot of programs, both British and American. We were completely sucked in by Survivors and are continually impressed by how unpredictable and exciting each episode is. We watch "24" and have enjoyed other edge-of-your-seat shows like "Alias" and "Prison Break" in the past, but we agree that Survivors is usually more entertaining than any of them.
Perhaps the total number of viewers in the UK hasn't been up to hopes, but maybe if demographics (average income, education level, etc.) are taken into account, PLUS you add in American viewers (via BBC America), the BBC should reconsider its decision and renew this riveting program for a third season (or "third series," as they call it) and beyond.
The premise of the show seemed fairly mundane: a virus wipes out most of humanity and we view the struggles of a ragtag bunch of survivors. But, despite a few logical lapses, the execution is excellent and the program is exciting to watch. Most of the characters are very well realized and acted, and the plot (episode-by-episode and ongoing too) is much better than one would expect. We look forward to watching the program every week.
They brought Doctor Who back (and we love it). Survivors deserves at least another few seasons -- especially given the miniscule 6-episode British seasons, which are about 1/4 as long as seasons of most American programs!