How do we know her fanbase shrunk?
An example is Diana Vickers. Her vote was very consistent (always around the 15% mark) but when it came to the semi final, the 15% wasn't good enough to get her to the final. Her fanbase didn't shrink, it just didn't grow.
Of course, none of us will know if Janet's fanbase grew, stayed consistent or shrunk until the final but I wouldn't be so quick to judge.
They've been trying to knock her fanbase down for weeks so tonight was hardly a surprising result. Always giving her an early slot, a few rubbish song choices, harsh and hypocritical comments... It was bound to work sooner or later - it always does.