This is an old question but worth mentioning. So you saw a dot in front of a tweet but don't know why it is used?
If your tweet starts with @ twitter thinks it is a conversation tweet and doesn't show this in news feed (your followers will not see this kind of tweets on home feed). Only those persons'll be able to see the tweet who follow you and the person which is being mentioned in the tweet, but it'll be available on your profile page and everyone'll be able to see it from there.
When I should use dot (.) in tweet?
If you tweet something that could be useful for your followers while mentioning someone at the start of tweet, you should add period (.). Here is an example:
.@username To love and be loved is like feeling Sun from both sides. #quote
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When I shouldn't use dot?
When you have personal tweet then you shouldn't use dot (.), twitter will limit its visibility because it is conversation tweet.
@username We are going to beach. Wanna go with us?
Tweets having "@" at start are considered as conversation tweets, so you can trick twitter by putting something else instead of dot(.) too.
Hello @username! "Do not spoil what you have by desiring what you don't have."
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