Does your bacteremia level increase if a disposable dental flosser is reused?

Probably not, but...: Although i'm unaware of studies that address this question, the intuitive answer would be "no". The bacteria count under the gum is so high that any contribution to total biomass from a piece of floss is unlikely to be significant. The magnitude of bacteremia would be more correlated with the amount of bleeding, which would reflect the extent of the breach in the wall of the vascular compartment.

Doesn't decrease: It doesn't necessarily increase but it isn't going to decrease.