- Is it possible to see how many users (especially experts) are online? or I should find it by checking user by user? It seem to me that nobody respond to my recent posts.
- Where can I find in my activity profile the list of posts that I follow them?
- Is it good feature that some users post a comment in other users profile and not under the posts? and what about "following the other users"? for example most of time I search for answers of some experts to read and learn from their answers.
Regarding your questions:
There is no dedicated feature to see how many users are online (but see Martin's comment for some ideas); it is also not exactly clear to me what it would mean. Generally, the site is more designed as an asynchronous medium, not for real-time or near real-time interaction.
The follow feature is very new. As far as I know such a tab does not yet exist but will be added. See the announcement The Follow Questions and Answers feature is now live across the Network Point 1 at the end.
The historical idea of SE is that it is centered on the content not the individual users, historically the idea that this is not a social network site was very strong. This is why there is no private messaging or comments on user pages, or anything like this. Over the years SE opened up a bit to the idea of having such features, so maybe it will come to exist. I would not exclude that now that the follow feature exists it is generalized down the road to user profiles, but that is speculation. If you want to follow a user that is still possible via RSS feeds. Look for the bottom of the page for the link to the feed.