Summary of significant accounting policies (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010
Revenue recognition
Obligations related to the earned fees and collection of the related receivable	$ 0
Advertising, marketing and promotion costs
Capitalized advertising production costs	0	0	0
Advertising expenses	2,374,000	1,393,000	579,000
Assumptions that were used to calculate the grant date fair value of employee stock option grants
Expected term	5 years 10 months 24 days	6 years 4 months 24 days	6 years 1 month 6 days
Expected volatility (as a percent)	48.90%	49.80%	67.60%
Risk free rate of return (as a percent)	0.90%	2.40%	2.50%
Dividend yield (as a percent)	0.00%	0.00%	0.00%
Stock-based compensation expense recognized for non-employee	$ 372,000	$ 515,000	$ 0
Dividends declared on common stock (in dollars per share)	$ 0.00
Dividends paid on common stock (in dollars per share)	$ 0.00
Number of dividend payouts anticipated in the foreseeable future	0
Platform service arrangements | Minimum
Revenue recognition
Term of the arrangement used to determine revenue recognition	2 years
Platform service arrangements | Maximum
Revenue recognition
Term of the arrangement used to determine revenue recognition	5 years
Wholesale partner arrangement | Minimum
Revenue recognition
Term of the arrangement used to determine revenue recognition	3 years
Wholesale partner arrangement | Maximum
Revenue recognition
Term of the arrangement used to determine revenue recognition	15 years