Do induction chargers waste electricity? Do induction chargers use more energy than traditional chargers while plugged in to the mains but not charging a device?
 A: If by 'induction charger' you mean only the basic circuitry less any circuitry to improve efficiency, then the answer is, "Yes, they are typically notably less efficient even when a charged abject IS above them". However, they can be made pretty efficient if 1) You mechanically limit the position and orientation of the charged device, 2) They include a circuit to detect the presence of a device to charge, which removes all but detection power from the primary if its not there, and 3) the primary and secondary inductors, cores, frequency etc. is adjusted for good efficiency given the various dimensions, charge rate, etc.  
A: When not charging a device, some of them have zero power consumption, or that's at least the claim made by NXP when they presented their new chip for the Qi standard, two years ago. It uses near-field communication, allowing the chip to be activated by a NFC enabled smartphone: the smartphone supplies the energy, comparable to how RFID tags are read. 
The ones that actively detect whether a device is present can in principle have very low stand-by power. 
A "properly designed Qi wireless charger" uses no more than 0.1 mW according to this source.
An electric toothbrush charger doesn't detect anything, it is basically an open core transformer  with the secondary coil in the toothbrush. Energy loss will be greatest when the battery is fully charged, but will still be considerable when no brush is present. 
The same is true for all chargers without control/detection circuit. I have an empty flashlight charger plugged in, and it feels quite warm, in contrast to the unused power supply for an external HD next to it. Smaller devices (lower maximum power) are typically less efficient (true for transformers in general), and with the possible exception of intensive use (several hours per day) these devices will waste (much) more energy than they supply for charging. But because they are so low power, the total amount will still be minor, less than your internet router or modem. 
These coils also cause reactive power, which you don't (directly) pay for (it's not actually used, and the electricity meter doesn't register it), but does cause joule losses in the transmission lines. But here the amount is negligible compared to total energy use (or to other losses).
A: It depends on how well the cheap Chinese knock-off charger you use works :-) but the general answer is no, they do not.
Most wireless chargers are cleverer than a dumb charger wart. They turn themselves off and only use power when a phone is detected. This means they can use less than 0.01W of power (according to NXP that makes the Qi chips). Obviously when charging more power is wasted, but I hear its about 70% of what you'd get from a wired charger. 
This article says that a wireless charger uses 14.4 Wh per year in standby. (for reference an iPhone uses about 16Wh to charge fully)
A 2012 study at Lawrence Berkeley National Laboratory found the average wired charger uses 0.24W, that's 2.2 kWh per year by comparison.
