Tesco is the clear winner in the online grocery market, in fact it takes almost 50p of every £1 spent on food shopping on the internet.This virtual monopoly is set to continue not just because of spending on fulfilment, dark stores, distribution, stock and offline marketing but due to its online visibility through organic search and a very visible well-structured website.Last week Graham Charlton of Econsultancy asked "are supermarkets missing out on SEO opportunities?"Yes is the easy answer for most, but across all food-stuff searches Tesco really is bucking the trend and hoovering up 50% of all online grocery spend.But is Tesco really good or are the others really bad?What is the the online grocery market worth? Over 94% of all groceries are bought in bricks and mortar stores. For any other statistic like that there would be little fuss or even attention for the other 5-6%.However, the online grocery sector equates to £5.6bn (compared with the whole of the grocery market being worth £157bn). In terms of revenue, there is serious land grabbing to be had.The online share of the grocery market is minuscule compared to other sectors within retail.Books, DVDs and music have long performed well online, perhaps partly due to the Amazon effect, and the relatively low brand loyalty to the store when purchasing such products.For music and videos we purchase 80% of goods online as opposed to offline. The whole of online retail averages 11.7% of all sales in the UK.