Does Vevo have a problem with falling viewer numbers? That’s one conclusion from comScore’s latest US Online Video Content Properties chart.

According to comScore, Vevo attracted 52m unique viewers in the US in February 2012, and they watched just under 697m videos, spending 60.5 minutes each on the service on average.

Music Ally’s analysis of comScore’s monthly charts for the last year (right) appears to show a worrying pattern for Vevo. Its unique viewers climbed steadily from 49m in February 2011 to a peak of 63m in June 2011, but then slid month by month to a low of 51.5m in January 2012, before reviving slightly in February to 52m.

The caveat: comScore’s figures don’t include mobile access to Vevo from its apps – we’d be interested to see if the growth in popularity of the apps is responsible for the online decline. We’re following this up with Vevo.

The latest comScore chart also claims that Google video sites (i.e. YouTube) attracted 147m unique viewers in February, and that they watched 16.7bn videos – down from 157m viewers watching 21.9bn videos in December.