How much escrow/ Multi-signature transactions are done? I have read here about the possibility of Multi-signature transactions. Is there a statistic available how much of these are done?
Blockchain doesn't currently offer that and the only way to construct a transaction like that is to build it manually.  So as a result both the sender and each recipient must have l33t skills.

This capability will come, it is just that since that was introduced, scalability had to take priority over new features for most Bitcoin development.

They happen but rarely.  Here's an example of Bitcoin address used recently (March 5, 2013) in an escrow transaction:

,- http://blockchain.info/address/13MH4zmU4UT4Ct6BhoRFGjigC8gN9a9FNn