I’d recommend against using an automated system to welcome new followers.
Whenever I follow someone and I get a automatic “welcome” direct message from them, it annoys me and I lose respect for the person. The biggest reason automated DMs don’t work well is that in many cases, the person sending the automated DM will not be following the user back, so they can’t reply to the DM. It’s frustrating to receive a message and not have the ability to reply.
Users don’t expect to get a welcome message, and I don’t feel that it is necessary. An automated and non-personal message can have the opposite effect you’re hoping for.
If you do decide to send automated DMs, I strongly encourage you to do something simple such as “Hey, thanks for the follow.” DO NOT use it as a way to plug yourself or get them to click a link. I’ve had a few people send an automated DM with a message along the lines of “Thanks for the follow! Check out my blog here: http://…” – I instantly unfollowed them. It makes users feel like they’ve just subscribed to spam.
When a user follows you, they’re telling you they’re interested in you and your content, so they feel like they’re giving you an endorsement. Sending them something automated or spam-like can feel like a slap in the face.
Instead of an automated system, try selecting a few interesting new followers and tweet to them saying “Hey, thanks for the follow!” and perhaps add something personal such as “You’re from Boston? Awesome! I went to school in Boston! What do you do there?” It engages new followers and establishes an authentic connection between you and your audience. You’ll be more likely to find out interesting facts and make some interesting contacts, and you’re less likely to lose that new connection.