Despite not even being registered...
If you'd already established yourself as a user on the forum, I'd have said, "Why not?" But since you are not already a known-quantity over there, I advise caution. As a default, the standards we have on this site seem good:
Post good, relevant answers, and if some (but not all) happen to be about your product or website, that’s okay. However, you must disclose your affiliation in your answers.
If a large percentage of your posts include a mention of your product or website, you're probably here for the wrong reasons.
Now, other sites have other rules. Some are geared toward all sorts of promotion—especially self-promotion. Others are even more protective than we are. In any case, it doesn't help promote the site if "spreading the word" looks like spamming. Follow what ever guidelines the site you are posting on provide.
MobileRead's guidelines read:
5. Advertising and Promotion
Please post in our Flea Market section if you want to buy or sell a reading device (or other item relevant to mobile reading) for your personal use only.
If you have something to promote – e.g. a new book, a new reading device, or other item relevant to mobile reading, then you may announce it, and respond to questions, in the Self-Promotions forum. Please read and follow the additional Promotion Posting Guidelines. Members who abuse these guidelines may be treated as spammers.
I don't know how promoting Ebooks.SE would be labeled. It would probably be best to ask first.