7. Intangible Assets

Intangible assets consist of the following (in thousands):

	March 31, 2018
	Gross		Accumulated Amortization			Net		Weighted Average Amortization Period (Years)
Promotion service rights	$	22,492	$	(4,991	)	$	17,501		5.8
Developed technologies		12,187		(5,656	)		6,531		2.8
Customer relationships		11,660		(6,979	)		4,681		4.1
Data access rights		10,801		(3,129	)		7,672		4.1
Media service rights		6,383		(1,849	)		4,534		4.1
Domain names		5,949		(4,839	)		1,110		1.1
Trade names		1,767		(500	)		1,267		3.2
Patents		975		(782	)		193		4.2
Vendor relationships		890		(890	)		—		—
Registered users		420		(257	)		163		1.9
	$	73,524	$	(29,872	)	$	43,652		4.5

As of March 31, 2018, and December 31, 2017, the Company has a domain name with a gross value of $0.4 million with an indefinite useful life that is not subject to amortization.

	December 31, 2017
	Gross		Accumulated Amortization			Net		Weighted Average Amortization Period (Years)
Promotion service rights	$	22,492	$	(4,252	)	$	18,240		6.1
Developed technologies		12,187		(5,013	)		7,174		3.0
Customer relationships		11,660		(6,547	)		5,113		4.3
Data access rights		10,801		(2,666	)		8,135		4.3
Media service rights		6,383		(1,575	)		4,808		4.3
Domain names		5,949		(4,689	)		1,260		1.3
Trade names		1,767		(401	)		1,366		3.4
Patents		975		(769	)		206		4.5
Vendor relationships		890		(890	)		—		—
Registered users		420		(232	)		188		2.2
	$	73,524	$	(27,034	)	$	46,490		4.7

Amortization expense related to intangible assets subject to amortization was $2.8 million and $2.4 million during the three months ended March 31, 2018 and 2017, respectively. Estimated future amortization expense related to intangible assets as of March 31, 2018 is as follows (in thousands):

	Total
2018, remaining nine months	$	8,629
2019		10,379
2020		8,962
2021		7,104
2022		4,408
2023 and beyond		3,817
Total estimated amortization expense	$	43,299