also bitcoin network should not be a place for advertisement!
well, OP_RETURN exists to allow people to insert arbitrary data into confirmed transactions, and that's mainly because it would be possible to do so anyway, so making a feature out of it (albeit not at all accommodating due to the 40 byte limit) was a compromise
OP_RETURN limit is 80 bytes (83 bytes if you include the overhead). But the limitation only applies during relaying transaction. Pool/miner could create coinbase transaction or include their own transaction with OP_RETURN with far bigger size, which means it can be used for more complex advertising/labeling.