Last year DropBox made $240m from from 2m paying customers (out of 50m users). So about $120/customer. DropBox's valuation is around $5-$10bn, around 20-40x revenue. Which given their growth rate seems a reasonable revenue multiplier.
Over the same period Evernote made $18m from 750k paying customers (out of 20m users). So about $24/customer. A $1bn valuation would give a multiplier of 50x.
LinkedIn is currently trading at 45x revenue - a figure that's been relatively stable for a while.
So arguably 50x is high but not ridiculously so, if Evernote manages to double their customer base over the next couple of years (or their revenue per customer) which isn't unfeasible, it'll look cheap at that price.
(Given Evernote costs $45/year that presumably means most of their customers signed up relatively late in the year resulting in the average revenue per customer being $24/year, so even if they only managed to keep their current customers they'd make 34m this year bringing their multiplier down to 30x)