These types of letters do get sent to other companies. Here's one from Franken to some obscure app company from February:

http://www.franken.senate.gov/?p=press_release&id=2699

Here's one to Google from Markey:

http://www.washingtonpost.com/business/technology/googles-pl...

Here's one to the FTC about Facebook from Warner:

http://www.warner.senate.gov/public/index.cfm/pressreleases?...

Here's one from Schumer to Zuckerberg:

http://www.schumer.senate.gov/record.cfm?id=324221

Here's one from Blumenthal and Franken to Social Intelligence (never heard of them before):

http://www.blumenthal.senate.gov/newsroom/press/release/blum...

I found all these by Googling something like "senator letter [company name] privacy". Most don't get covered here because they are typically routine pandering by politicians that doesn't change anything. (As is this letter IMO.)