There has bit quite a bit of controversy around non-animal sources of vitamin B12 those last years and even decades but recently some research has put a bit of perspective into the hype around vitamin B12 deficiency in vegetarians, let alone vegans:
Most scientific sources agree that the recommended dietary allowances (RDA) of this vitamin is between 1 to 2,5 micrograms – just as a reminder of your science class: a microgram is one-millionth of a gram (!) – therefore unless you suffer from severe malabsorption or any other life-threatening disease it is very unlikely that you suffer from a vitamin B12 deficiency.
Most of us, who eat commodity food now and then, like breakfast cereals, sandwich bread, fortified flour, cereal drinks (non-diary milk), are exposed to vitamin B12 as well as other vitamin supplementation, without even looking for it.
Those who stay away from industrially packaged food can still find enough vitamin B12 in Spirulina, one of the very best sources, as well as in other seaweeds, some fermented soy food and “traditionally" in friendly bacterias found in the ground – yes the soil – if you don’t scrub your organic veggies too obsessively.
This is more than enough for getting the “huge" amount of a millionth part of a gram!
But wait-a-minute, didn’t you read somewhere the vitamin B12 found in Spirulina and other algae is touted pseudo-vitamin B12, or B12 analog and is non-assimilable, and moreover hinders the real and good B12 from binding to the cells who transport it to your organs?
Well, false alarm, recent scientific studies finally seem to prove exactly the opposite, and the misconception was due, at least in part, to the methods used for determining the availability of pseudo B12 for the human body.
The B12 analog is not a substitute form of vitamin B12 but in fact one of the most important factors of biosynthesis of several processes, one of them the synthesis of methionine, which enable the production of active B12 (methyl-cobalamine).
Here are some scientific data:
In 2010, Kumudha and his team  identify and quantify the methylcobalamin (active form of vitamin B12) Spirulina platensis as follows:
They quantify this methylcobalamin by microbiological test and chemiluminescence test and find respectively levels of 38.5 μg (+/- 2 μg) and 35.7 μg (+/- 2 μg) for 100 g of dry biomass.
Based on the results obtained in this last study, we can deduce that a daily dose of 3 g of Spirulina (6 cp of 500 mg) give 1.1 μg of active vitamin B12.
We can conclude that 3 g of Spirulina would provide about 44% of RGAs active vitamin B12. The recommended daily intake (RDA) of vitamin B12 is between 1 to 2.5 μg.
Is the adeninylcobamide (pseudo-) form of vitamin B12 really inactive?
As indicated above, vitamin 12 acts as a cofactor in some reactions.
Methylcobalamin (an active form of vitamin 12) is a cofactor of methionine synthase, an enzyme that allows the biosynthesis of methionine from homocysteine.
A vitamin B12 nutritional deficiency can lead to an increase in homocysteine levels in the blood. However, a too high homocysteine level can be a risk factor for vascular accidents.
We have also seen that there is a form of vitamin B12 called pseudo-vitamin B12 that would not be active for humans, as suggested by previous years of research.
However, recent data reveal that pseudo-vitamin B12 may not be as inactive as it was supposed to be.
In his 2009 thesis , Sandro Roselli reports the ability of certain microorganisms to use pseudo-vitamin B12 as a cofactor for all their B12-dependent enzymes.
He also emphasizes that a multitude of recent studies prove that several anaerobic organisms produce some non-negligible amounts of pseudo-vitamin B12 … / … It appears that these compounds long considered as substitution forms (pseudo vitamin) are in fact major cofactors for many organisms.
In 2010, the results of the research of Takinoa and his team  go in this direction too. Indeed, they demonstrate that Spirulina Platensis cells are able to use the pseudo-vitamin B12 synthesized as a cofactor of methionine synthase.
 Kumudha A. & al.: Purification, Identification, and Characterization of Methylcobalamin from Spirulina platensis. J Agric Food Chem Aug 2010.
 Roselli S. : Génomique fonctionnelle de la dégradation microbienne du chlorométhane. Thèse soutenue publiquement le 15 décembre 2009 – Université de Strasbourg.
 Tanioka Y. & al.: Methyladeninylcobamide functions as the cofactor of methionine synthase in a Cyanobacterium, Spirulina platensis NIES-39. FEBS Lett. 2010 Jul 16;584(14):3223-6. Epub 2010 Jun 17.