I have always said that it is very important to acknowledge your new connections on networks like Facebook and LinkedIn- the ones where you manually accept. I have also said that for networks like Twitter or Google Plus it is not so essential to acknowledge your new followers because it can be challenging to keep up by doing so. However, I am starting to disagree with that belief. Now, in regards to Google Plus, I still am not overly concerned for that reason. However, for Twitter I recommend that you do and let me expand on this further.
If you are consistently sharing good quality content on Twitter, and interacting with other followers- it will be easily noticed. Therefore, you will be attracting new followers on a consistent basis and most of them will be of decent quality. These individuals you certainly will not only want to follow back, however you will want to establish a relationship with them. Sure, you can retweet their tweets right away which they definitely will like. By all means do that. However, they will be more inclined to engage with you more and to connect with them on a more personal level if you welcome them right away. As you know, this can lead to them becoming a client/customer or they may refer others over to you who are looking for what you can offer.
I do not recommend welcoming them in a personal message on Twitter. Many do not even bother looking at their messages. You also do not need to welcome new followers one by one. In fact, if you end up with 24 new followers in one shot- you are not going to send 24 individual tweets welcoming each and every one of them. That will annoy your other followers. You can welcome them in groups. For instance, if you are able to fit the handles of 8 new followers in one tweet- then do that and send 3 different tweets instead. Be sure to type in #Welcome in the beginning as well. You will have 9 characters of 140 used (1 used for the space after the word). Here is an example of how to welcome your new followers in one shot.
— Miriam Slozberg (@msmir) October 2, 2014
In this particular welcome tweet, I was able to acknowledge 10 new followers.
Your new followers will love this. They may even retweet this and/or mark it as a favorite. You however do not need to do this for junk accounts. Some of those junk accounts may be bots or spam accounts. You can definitely ignore those. However, for the good quality accounts that follow you- be sure to send them a nice warm welcome even if you decide not to follow them back right away (however, it is recommended that you do because that is another way to show interest in what they are sharing).