Facebook, Yahoo, and Microsoft just lent their support to AT&T’s proposed merger with T-Mobile, saying the union could help meet a coming crunch for wireless broadband capacity.

In a letter to the chairman of the Federal Communications Commission filed late Monday, several Silicon Valley giants said a voracious appetite for smart phones, tablets and the apps they provide need more wireless spectrum to smoothly connect people online on the go.

AT&T and T-Mobile can together “leverage a larger network of cell sites allowing greater reuse of spectrum and increasing the wireless broadband capacity of the network,” the companies wrote.

The letter was also signed by Blackberry maker, RIM, Oracle, Avaya, Broacase and Qualcomm.

Many of these firms partner with AT&T or T-Mobile. Microsoft carries its Windows smartphones on AT&T and T-Mobile’s networks. Qualcomm and Avaya provide devices and networking equipment for AT&T too.

AT&T executive vice president James Cicconi was in Silicon Valley last month to talk up the merger with industry leaders.

The support comes amid a barrage of opposition from wireless competitors such as Sprint Nextel, Cellular South and Leap Wireless, who all argue that the $39 billion merger would lead to less competition and make it harder for small carriers to get the best devices — such as the iPhone — on their networks.

Related Stories:

AT&T ramps up lobby ahead of merger

Lawmakers skeptical over AT&T-T-Mobile merger

Senators grill AT&T-T-Mobile over merger