I want to assume that 'smb' means 'somebody' in short-hand writing. It could mean the following.
1. Somebody has looked at your research interests and want to know your latest updates in those areas.
2. It could mean that somebody know you personally and just want to be informed about your progress in publishing and related-academic activities you post to RG.
3. It could be that younger researchers are seeking mentors to pattern their research writing after. Your writing style based on your work uploaded to RG may be interesting to them as could serve as a template that they may want to follow. For this reason, they could choose to follow.
4. Researchers that you have cited may choose to follow you if you had followed them initially.
5. If you have recently published a paper similar to what someone else is working on, the author may choose to follow you.
6. Others may follow you because you are in the same department or university as themselves. If you have attended the same academic conference, they may follow you as well
7. If your name is popular and established in your field, others will follow your update.
Only those who have read the abstract and what to see the content will be interested to download the full text. However, they have the option on RG to request for the full-text from you.