YSK: "All natural" bamboo fabric is almost always a semi-synthetic fibre called rayon which can be made from many different types of wood and plant pulp. Depending on the production method, rayon may also release harmful solvents which can injure workers.
"Textiles labelled as being made from bamboo are usually not made by mechanical crushing and retting. They are generally synthetic rayon made from cellulose extracted from bamboo." - Bamboo textile - Wikipedia
Also, many of the claims about bamboo fibre properties are also false. Once processed in to rayon, the unique properties of bamboo are no longer present. Products with claims such as "natural anti-bacterial, UV blocking, instant moisture evaporation." don't stand up to investigation. This is because "bamboo viscose rayon is indistinguishable from normal viscose rayon".
Also, many of the 'comfort' properties that are often sold as an advantage of bamboo fabric have nothing to do with the bamboo input material. The final texture of the fabric being dependent on the rayon processing technology.
"Rayon is a versatile fiber and is widely claimed to have the same comfort properties as natural fibers, although the drape and slipperiness of rayon textiles are often more like nylon. It can imitate the feel and texture of silk, wool, cotton and linen..." Rayon - Wikipedia
Finally the production of viscose rayon may release harmful solvents that have serious toxic effects on those exposed. WHO: Air Quality Guidelines - Carbon disulfide
There may be environmental benefits to using bamboo pulp for creating rayon, including it's ability to grow in difficult conditions and fast growing properties. There are also different ways of producing rayon that reduce environmental and worker impacts. As with all things, the details matter.
TL;DR: Bamboo fibre fabrics are nothing special, they are in fact a fabric called rayon, which can be made from a wide range of wood and plant pulps. The production process retains almost no properties of the raw material, and some processing methods are harmful to workers and the environment (while others are not). It's not necessarily the case that all rayon or all bamboo fibres are bad, however YSK the details about how your particular fabric has been made and consider the voracity of: 'all natural', 'unique material', 'eco-friendly' marketing features before making your purchasing decisions and paying extra for bamboo fabric. If you like your bamboo socks, go ahead, buy them! I don't care, I'm not your mother!
OP's personal note: I was comparing two different bamboo socks yesterday, not because of eco-friendly tags, just because I liked the designs on them. I called the first manufacturer to ask some questions about their production process and they refused to answer any questions at all about their production processes.
The second (bamboo) sock manufacturer claimed that they used a "100% closed loop production process that releases no solvents or chemicals to the environment".
Perhaps they were feeding me bullshit, but it's at least it was a step better than the guys who refused to answer any questions at all.