Can I use informal sources (particularly online sources)?
If your paper is on "real-world implementations of Bluetooth LE" then mailing lists on implementations of Bluetooth LE and manufacturers' documentation of Bluetooth LE products are your primary sources.  This is akin to e.g., a historian using Napoleon's informal personal correspondence as a source for information on his world.

Of course, you should consider where these sources come from in how you use them. If you read on a mailing list or forum, 


  Bluetooth LE is the worst thing ever. Mine never works!


you would not use it as a factual source to conclude that Bluetooth LE is terrible. You certainly could use it as a source to indicate that some users have experienced frustration with early implementations of Bluetooth LE. (I cite bug tickets often in my own papers.)

Bad use of informal source:


  Bluetooth LE is not a good technology [1].


[1] "Let's all complain about Bluetooth LE here," Bluetooth User Forum, posted April 1 2104, http:/bluetoothforum/lets-complain, retrieved April 25 2014.

Good use of informal source as a primary source:


  Bluetooth LE users have expressed frustration with the technology [1].


[1] "Let's all complain about Bluetooth LE here," Bluetooth User Forum, posted April 1 2104, http:/bluetoothforum/lets-complain, retrieved April 25 2014.