FR24 claim to use "FAA data", what it means I do not know, however the recorded altitude seems to be corrected for QNH (which is not the case with ADS-B raw values)
Other than ADS-B, sites like FR24 get a "near real-time" data feed from the FAA (called the ASD feed). This feed contains flight plan and position data (including lat / long, ground speed) for all IFR aircraft in the US air space.