How much weight can a light bulb socket hold?
Not a great idea.


Electrical devices are designed and approved to be used unmodified. Adding parts or using them other than as intended voids the UL listing and, more importantly, poses risks of fire or electrocution.
Attaching something to a plug in device at or near the plug runs the specific risk of shorting out the spade terminals, risking the above noted fire or electrocution.
The weight that could be supported (but shouldn't be) would depend on whether the plug in socked was inserted horizontally or vertically (hanging down), the latter being much less reliable than the former, which is also not reliable.
The actual strength of the hold is a factor of the tension created by the contact points within the wall socket itself. This is variable across brands, types of outlets, and changes over time and usage. It is impossible to predict whether a socket will hold a regular plug, let alone extra weight attached to a device that already puts a downward strain on the grip. Another reason not to do it.
Finally, plug in lamps are varied in their quality and hence their strength, again making a load prediction nigh on impossible.
Don't do it.