It's no secret that MMOs have started to see a larger and larger share of the market, but the actual amount is always a difficult number to pin down. Gamasutra recently reported on a study by TNS and Gamesindustry.com, a pair of market research firms, which found that about 20% of all US game spending goes to either MMOs or online game portals. That's a fifth of all spending in the country, and with the amount of overall spending in the US that's a rather significant figure.
Of course, MMOs are still small fry compared to, say, the console budget, which dominates at nearly three-fifths of the overall spending. However, it's worth considering that MMO spending is frequently going to be a $15 monthly subscription rather than a $50 one-time purchase -- a significant disparity in terms of overall budget. It's also interesting to note that MMOs, once a small subset of the overall PC gaming market, are gaining ground as a market demographic unto themselves. The full report can be found here, with further information on the methodology of the study and the preferred payment methods of the respondents.