As I said before, this is NOT true in a general statement. There is an important factor called intrinsic factor that deminishes over time, with age, with diet, with disease, and with medications. This factor is REQUIRED for your body to metabolize B12. Without this factor, you cannot process B12. I know this from experience, as the medication I was on before I became insulin dependent was metformin, and metformin has been found (and did in my case) to reduce intrinsic factor. I could take 10,000mcg of B12, and register NOTHING in my blood. I could not process B12. I started B12 shots daily for two weeks, then twice a week for a couple weeks while I discontinued the metformin. Within a couple months, I was able to process B12 again, and now can take it orally.
However, there are many factors that contribute to deminished intrinsic factor, and they are finding it more common now. But unless you know you have a problem, then oral is fine - but if you suspect, like I said, take a bunch orally for a few days, then have a blood test to see if you have high levels.
But I can say this - when I was taking injections (100mcg daily for two weeks, 100mcg twice a week, etc), I was HUNGRY all the time, and really felt a euphoria. I do NOT feel that way on oral administration, maybe just a little. But again, everyone is different.