Suspicious Permissions And A Safe Repository? I have found older version of Facebook For Android app here which was hosted on geekfiles.in

When i wanted to install 1.8.1 and checked permissions, the following arose my suspicions:

Services that cost you money:
Send SMS messages

Your messages:
edit SMS or MMS, read SMS or MMS, receive SMS

Your accounts:
act as an account authenticator, manage the accounts list

Phone calls:
read phone state and identity

Your accounts:
discover known accounts

Storage:
modify/delete SD card contents

Can unscrupulous people inject malware into APKs? If yes, what are trusted repositories for downgrading apps to older versions?

Is this repository trusted source?
As a general rule, I don't trust any "freeware" or "shareware" site that has not specifically been recommended to me by someone I know and trust, and because these sites aren't my thing I can't either recommend or condemn this specific example. I can answer part of your question, though, and say an emphatic yes, unscrupulous people can inject malware into APKs. Installing an APK from an unknown source is generally a very bad idea.

The permissions you mention, though, are actually the same that the legit Facebook for Android app asks over in the Play store (see the "Permissions" tab). Permissions alone are a bad judge of an APK's potential for harm. In this case, personally, I would say the risk of an unknown source far outweighs the app's permission requests.