OK, this may take me a while. Bear with me, folks, and I'll try to provide some answers.

First off, ethanol and methanol are closer to being carbohydrates than hydrocarbons, because they contain oxygen.

There are bacteria that can consume methane (it is slightly soluble in water), but they have to start out by using energy to convert it to methanol before they can extract any energy from the reactions. So to make methanol from methane in a controlled fashion requires energy (the C-H bond is very stable), and it is only by oxidising methanol that the bugs can obtain energy from the chemical reactions. If methanol is available, they will tend to favour that.

There are fungi that can metabolise long-chain hydrocarbons such as are found in crude oil. However, I do not know very much about these organisms. I believe they metabolise hydrocarbons in order to reduce their toxicity and not primarily to obtain energy. This ability to reduce the toxicity of toxic hydrocarbons gives these fungi an adaptive advantage in certain environments.

The point made in earlier posts about hydrocarbons being poorly soluble in water is a valid one, but it is not the most important consideration. There are two other points that I think are more important:

1. Since carbon and hydrogen have very similar electronegativity, a hydrocarbon molecule is very uniform, or non-polar. It does not develop areas of small positive or negative charges because the electrons that make the chemical bonds between atoms (or, more accurately, the electrons that occupy the highest occupied molecular orbital, or HOMO) are evenly "spread" over the molecular orbitals in the molecule. A consequence of this is that the enzymes that catalyse metabolism would have nothing with which to "grab hold" of the hydrocarbon molecule. Enzymes are biological catalysts; they are protein molecules, and as such have to carry out their catalytic function using chemical groups that exist on the 22 or so amino acids in proteins (there are 20 a.a.s from which proteins are made, but sometimes some are modified afterwards). The enzyme must align the substrate molecule in the correct orientation for its catalysis to be effective. To do this, it needs the substrate molecule to have areas that are chemically different. The simplest way to do this chemically is by having heteroatoms such as oxygen in the substrate molecule. Because oxygen has a high electronegativity, the electrons in bonds adjacent to O atoms spend more time near the oxygen than near, say, the C or H atoms. Therefore, the O atom develops a slight negative charge, making the molecule polar. This difference is exploited by an enzyme molecule to control the orientation of the substrate molecule and hence facilitate catalysis. Simply put, it is easier for an enzyme to catalyse a reaction with a carbohydrate molecule than with a hydrocarbon molecule.

2. Metabolism is often compared to burning, but there are important differences in the chemistry. The breakdown of carbohyrate into CO2 and water is done in a gradual and controlled fashion. If it were to happen all at once, the energy would be lost (i.e. not captured in a useful form) and might actually boil the cell. Some very sophisticated systems have evolved to capture the energy of the oxidation of carbohydrate and "store" it in a molecule that can be moved around. This molecule is ATP (adenosine triphosphate). The breakdown (hydrolysis) of the phosphodiester bond between the second and third phosphate groups of ATP has a high Gibbs free energy associated with it. This can be coupled to the various processes for which living things require energy, and hence used to do work (e.g. to move other molecules around, to make DNA, to make structural molecules and so on).

There's more. We do metabolise molecules that are nearly hydrocarbons. The fats mentioned by kzb in post #11 are triacyl glycerols. Glycerol is simply three carbons, each with an alcohol group (OH) attached (the rest of the bonds are occupied by hydrogen atoms). The triacyl part will be three fatty acids. A fatty acid is a long-chain hydrocarbon (typically about 18 - 20 carbons, often saturated with hydrogen atoms, but sometimes having one or more points of unsaturation, meaning carbon-carbon double bonds) with a carboxylic acid group at one end. This acid group allows enzymes to get a hold on the molecule, and can also take part in useful, reversible chemical reactions. The acid groups can form ester bonds with glycerol (these are bonds that can be made and broken with only small changes in energy required so they are quite convenient for a system that requires them to be made reversibly) to form the triacyl glycerol molecule. Per unit weight, these provide more energy when broken down than carbohydrates (because oxygen is available from the air - carbohydrates are denser than fats because they contain oxygen, which has a higher molecular weight than carbon or hydrogen). Animals store surplus food as triacyl glycerol; plants store their food as carbohydrate because the do not need to carry it anywhere. Consequently, plants need to express fewer enzymes for their energy metabolism (not counting photosynthesis). Animals do store a small amount of carbohydrate in the form of glycogen. This is why breakfast is the most important meal of the day: first thing in the morning, your glycogen reserves are low, so your blood-sugar level will be low.

However (because nothing is ever that simple), not all of our tissues can metabolise fatty acids as a source of enegy. In particular, the brain requires glucose. It cannot obtain energy from any other source (which is why diabetics need to regulate their blood sugar level through diet or regular doses of drugs). One of my physiology professors once told us that the brain requires one Mars bar's-worth of glucose every day. So that works quite well for me... Anyway, because the brain requires glucose, it is easier for us to obtain that as carbohydrate in our diet (starch is a polymer of glucose) than to synthesize it from fats and oxygen.

One caveat here: I'm a little hazy in my recollection of lipid metabolism; it may be that we cannot synthesize glucose from fats, but I cannot recall any reason why this should be so. I do know that we can make glucose from other carbohydrates or from amino acids (with the excess nitrogen from amino acids being incorporated into urea and than passed from the body in urine).

So, although that was a bit long-winded, I think I've covered all the basics.

__________________
The quarrelsome oarsmen were rowing,
The great violinist was bowing;
But how is the sage
To tell, from the page:
Was it pigs or seeds that were sowing?