I see that many people have said this, but I think it's worth noting that, depending on the server*, this can be deemed as slightly more socially acceptable. I've received tells from people saying that they were not interested in joining an FC, but that they appreciated the fact that I spoke to them specifically via tell (no macros) versus sending out an unsolicited invite.
This isn't to say that a great number of players don't see it as rude, but in my experience, it isn't necessarily deemed as being socially unacceptable. I am sure this varies from server to server, however. Good luck!